[OnePlus 3 Questions & Answers] AlkaliV2:
People that want F2FS will lose their data converting their filesystems unless they back it up; no method listed in this thread gets around this. All the options listed keeps the users on ext4 because the official OTA does not convert your /data partition to F2FS as shown in the updater script code below from official 4.0.1 full firmware and update file.

getprop("ro.display.series") == "OnePlus 3" || abort("E3004: This package is for \"OnePlus 3\" devices; this is a \"" + getprop("ro.display.series") + "\".");
show_progress(0.750000, 0);
ui_print("Patching system image unconditionally...");
block_image_update("/dev/block/bootdevice/by-name/system", package_extract_file("system.transfer.list"), "system.new.dat", "system.patch.dat") ||
abort("E1001: Failed to update system image.");
show_progress(0.050000, 10);
show_progress(0.050000, 5);
package_extract_file("boot.img", "/dev/block/bootdevice/by-name/boot");
show_progress(0.200000, 10);
ui_print("Writing static_nvbk image...");
package_extract_file("RADIO/static_nvbk.bin", "/dev/block/bootdevice/by-name/oem_stanvbk");

# ---- radio update tasks ----

ui_print("Patching firmware images...");
ifelse(msm.boot_update("main"), (
package_extract_file("firmware-update/cmnlib64.mbn", "/dev/block/bootdevice/by-name/cmnlib64");
package_extract_file("firmware-update/cmnlib.mbn", "/dev/block/bootdevice/by-name/cmnlib");
package_extract_file("firmware-update/hyp.mbn", "/dev/block/bootdevice/by-name/hyp");
package_extract_file("firmware-update/pmic.elf", "/dev/block/bootdevice/by-name/pmic");
package_extract_file("firmware-update/tz.mbn", "/dev/block/bootdevice/by-name/tz");
package_extract_file("firmware-update/emmc_appsboot.mbn", "/dev/block/bootdevice/by-name/aboot");
package_extract_file("firmware-update/rpm.mbn", "/dev/block/bootdevice/by-name/rpm");
package_extract_file("firmware-update/devcfg.mbn", "/dev/block/bootdevice/by-name/devcfg");
package_extract_file("firmware-update/xbl.elf", "/dev/block/bootdevice/by-name/xbl");
package_extract_file("firmware-update/keymaster.mbn", "/dev/block/bootdevice/by-name/keymaster");
), "");
ifelse(msm.boot_update("backup"), (
package_extract_file("firmware-update/cmnlib64.mbn", "/dev/block/bootdevice/by-name/cmnlib64bak");
package_extract_file("firmware-update/cmnlib.mbn", "/dev/block/bootdevice/by-name/cmnlibbak");
package_extract_file("firmware-update/hyp.mbn", "/dev/block/bootdevice/by-name/hypbak");
package_extract_file("firmware-update/tz.mbn", "/dev/block/bootdevice/by-name/tzbak");
package_extract_file("firmware-update/emmc_appsboot.mbn", "/dev/block/bootdevice/by-name/abootbak");
package_extract_file("firmware-update/rpm.mbn", "/dev/block/bootdevice/by-name/rpmbak");
package_extract_file("firmware-update/devcfg.mbn", "/dev/block/bootdevice/by-name/devcfgbak");
package_extract_file("firmware-update/xbl.elf", "/dev/block/bootdevice/by-name/xblbak");
package_extract_file("firmware-update/keymaster.mbn", "/dev/block/bootdevice/by-name/keymasterbak");
), "");
msm.boot_update("finalize");
package_extract_file("firmware-update/NON-HLOS.bin", "/dev/block/bootdevice/by-name/modem");
package_extract_file("firmware-update/adspso.bin", "/dev/block/bootdevice/by-name/dsp");
package_extract_file("firmware-update/BTFM.bin", "/dev/block/bootdevice/by-name/bluetooth");
set_progress(1.000000);

You can see that all it does it overwrite the system partition with the new changes, writes the radio file, and flashes the extra firmware to bring you up to speed with nougat. If you want to do this right and be on F2FS you have to do it manually.