[Lenovo P2 Questions & Answers] Kontex:
At the moment I can not recommend to downgrade an unlocked P2 with Android 7. Found your Post and created a Batch file. It failes in createing and ereasing some steps. But the phone is starting with Android 6, now. I'll investigate the modem functions within the next days and post the results. Maybe QPST-Images are required for correct downgrading.

fastboot.exe flash partition "gpt_both0.bin"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:partition: not found
target reported max download size of 536870912 bytes
sending 'partition' (33 KB)...
OKAY [ 0.035s]
writing 'partition'...
OKAY [ 0.031s]
finished. total time: 0.076s

fastboot.exe flash bootloader "bootloader.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:bootloader: not found
target reported max download size of 536870912 bytes
sending 'bootloader' (4234 KB)...
OKAY [ 0.128s]
writing 'bootloader'...
FAILED (remote: partition table doesn't exist)
finished. total time: 0.164s

fastboot.exe flash logo "logo.bin"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:logo: not found
target reported max download size of 536870912 bytes
sending 'logo' (1907 KB)...
OKAY [ 0.072s]
writing 'logo'...
OKAY [ 0.068s]
finished. total time: 0.150s

fastboot.exe flash boot "boot.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:boot: not found
target reported max download size of 536870912 bytes
sending 'boot' (21677 KB)...
OKAY [ 0.539s]
writing 'boot'...
OKAY [ 0.322s]
finished. total time: 0.873s

fastboot.exe flash recovery "recovery.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:recovery: not found
target reported max download size of 536870912 bytes
sending 'recovery' (25265 KB)...
OKAY [ 0.604s]
writing 'recovery'...
OKAY [ 0.375s]
finished. total time: 0.987s

fastboot.exe flash dsp "adspso.bin"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:dsp: not found
target reported max download size of 536870912 bytes
sending 'dsp' (16384 KB)...
OKAY [ 0.393s]
writing 'dsp'...
OKAY [ 0.242s]
finished. total time: 0.646s

fastboot.exe flash system "systemchunk0.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (260121 KB)...
OKAY [ 5.972s]
writing 'system'...
OKAY [ 4.177s]
finished. total time: 10.159s

fastboot.exe flash system "systemchunk1.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (258079 KB)...
OKAY [ 5.922s]
writing 'system'...
OKAY [ 3.612s]
finished. total time: 9.546s

fastboot.exe flash system "systemchunk2.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (258077 KB)...
OKAY [ 5.876s]
writing 'system'...
OKAY [ 3.416s]
finished. total time: 9.302s

fastboot.exe flash system "systemchunk3.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (261716 KB)...
OKAY [ 6.034s]
writing 'system'...
OKAY [ 3.772s]
finished. total time: 9.819s

fastboot.exe flash system "systemchunk4.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (262033 KB)...
OKAY [ 5.988s]
writing 'system'...
OKAY [ 3.891s]
finished. total time: 9.891s

fastboot.exe flash system "systemchunk5.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (251510 KB)...
OKAY [ 5.771s]
writing 'system'...
OKAY [ 3.444s]
finished. total time: 9.227s

fastboot.exe flash system "systemchunk6.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (259097 KB)...
OKAY [ 5.902s]
writing 'system'...
OKAY [ 3.406s]
finished. total time: 9.320s

fastboot.exe flash system "systemchunk7.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (259098 KB)...
OKAY [ 5.903s]
writing 'system'...
OKAY [ 3.480s]
finished. total time: 9.441s

fastboot.exe flash system "systemchunk8.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (259098 KB)...
OKAY [ 5.899s]
writing 'system'...
OKAY [ 3.532s]
finished. total time: 32.879s

fastboot.exe flash system "systemchunk9.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (259100 KB)...
OKAY [ 5.983s]
writing 'system'...
OKAY [ 3.691s]
finished. total time: 9.687s

fastboot.exe flash system "systemchunk10.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (250251 KB)...
OKAY [ 5.751s]
writing 'system'...
OKAY [ 3.830s]
finished. total time: 9.593s

fastboot.exe flash system "systemchunk11.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (261655 KB)...
OKAY [ 5.981s]
writing 'system'...
OKAY [ 4.033s]
finished. total time: 10.026s

fastboot.exe flash system "systemchunk12.img"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending 'system' (64584 KB)...
OKAY [ 1.490s]
writing 'system'...
OKAY [ 0.878s]
finished. total time: 2.379s

fastboot.exe flash modem "NON-HLOS.bin"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:modem: not found
target reported max download size of 536870912 bytes
sending 'modem' (131072 KB)...
OKAY [ 3.053s]
writing 'modem'...
FAILED (remote: Do not allow to flash Bootloader image on Unlock device)
finished. total time: 3.118s

fastboot.exe erase modemst1
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:modemst1: not found
erasing 'modemst1'...
OKAY [ 0.037s]
finished. total time: 0.043s

fastboot.exe erase modemst2
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:modemst2: not found
erasing 'modemst2'...
OKAY [ 0.034s]
finished. total time: 0.040s

fastboot.exe flash fsg "fsg.mbn"
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:fsg: not found
target reported max download size of 536870912 bytes
sending 'fsg' (300 KB)...
OKAY [ 0.026s]
writing 'fsg'...
OKAY [ 0.029s]
finished. total time: 0.062s

fastboot.exe erase cache
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:cache: not found
erasing 'cache'...
OKAY [ 0.030s]
finished. total time: 0.035s

fastboot.exe erase userdata
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:userdata: not found
erasing 'userdata'...
OKAY [ 0.203s]
finished. total time: 0.208s

fastboot.exe erase customize
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:customize: not found
erasing 'customize'...
FAILED (remote: Partition table doesn't exist
)
finished. total time: 0.032s

fastboot.exe erase clogo
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:clogo: not found
erasing 'clogo'...
FAILED (remote: Partition table doesn't exist
)
finished. total time: 0.039s

edit2: OTA seems to work: http://phonedl.ota.lenovomm.com/dls/v6/P2a42_S048_161111_ROW_TO_P2a42_S062_170117_ROW_WC29E4CD37.zip