[Paid Software Announcement and Discussion] ChazzMatt:
OK... :)

With an UNlocked bootloader, YES on a Droid Turbo you CAN go back (downgrade) to any previous firmware you wish. Many have done it, to see if stock Kitkat or Lollipop uses less battery than Marshmallow, as they remember when they first got the phone.

With a LOCKED bootloader, no, you can't downgrade.

With an UNlocked bootloader, you flash the firmware version you wish with ADB -- but be sure to omit GPT.bin from the flash commands or you will bootloop.

Also, if you have TWRP and wish to keep that, also omit recovery.img from the flash commands.

This also applies to any of the Moto Quarks:

Droid Turbo XT1254/U.S. Moto Maxx XT1250,

Moto Turbo XT1225/"international" Moto Maxx XT1225.

The Droid Turbo XT1254 and U.S. Moto XT1250 are actually the exact same device -- with same FCC ID and CDMA/LTE radio bands. It's just Verizon wanted to pretend they had an "exclusive", when they didn't even have an exclusive in the United States. (Half a dozen U.S. regional carriers CDMA/LTE carriers sold the U.S. Moto Maxx.) Motorola renamed the XT1250 as the "Droid Turbo XT1254" to make Verizon happy.

Before Sunshine unlocked the Droid Turbo XT1254 bootloader in late 2015, many people bought the XT1250, unlocked the bootloader with free code from Motorola, installed TWRP, rooted, stuck in a Verizon SIM card and ran it on Verizon's network. The XT1250 U.S. Moto Maxx was basically the "dev" version of the Droid Turbo XT1254. ;) You can cross flash XT1250/XT1254 stock firmware, even, because they are the EXACT SAME device, with the same FCC ID.

The Moto Turbo XT1225/"international" Moto Maxx XT1225 (same phone, different countries) has the same specs as the XT1254/XT1250 -- except for internal memory (XT1225 only comes in 64GB) and different radio bands. (No LTE band 13, no CDMA -- but has LTE bands 5 & 17, plus more HSPA bands.) Thus, the Moto XT1225 has a different FCC ID than the Moto XT1254/XT1250.

The XT1225 and XT1254/XT1250 all share LTE bands 2, 3, 4, 7.

The XT1225 additionally has LTE band 5 and 17 (more bands used by AT&T).

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FCC ID IHDT56PK2

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The XT1254/XT1250 instead has LTE band 13 (Verizon LTE band), plus CDMA bands. While it does have some HSPA bands, there's not as many as in the XT1225.

FCC ID IHDT56PK1

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Think of it this way... Just like all LG G3 phones all over the world are the same phone series and have the same specs (except radio bands) , all the Moto Quarks are the same phone series. It's just Motorola confusingly called them different model names. Even two different model names for the EXACT same phone in the United States! The XT1254 came in 32GB/64GB configuration, while the XT1250 was 32GB only. The XT1225 (with different radio bands) came in 64GB only.

All the Quarks -- except the Droid Turbo XT1254 -- have easily unlocked bootloaders via free code from Motorola. I own three XT1225, run them on AT&T, and wrote a guide on unlocking the bootloader and rooting the XT1225 and XT1250. Only the XT1254 locked down bootloader is exclusive to Verizon. ;)

But thankfully Sunshine can unlock the bootloader on any Droid Turbo Verizon firmware for the past two years. Any firmware before that, you can update to firmware which can be unlocked.

After that it's YOUR phone. You can downgrade back to Kitkat if you wish.