I have a few devices I want to put back from custom roms to stock. On almost every stock rom source, however, there are dire warnings for some devices not to downgrade to stock versions from before your latest installed rooted rom version. For example, from Androidayos:
UPDATE: DO NOT DOWNGRADE back to the old stock firmware version. Once your Galaxy Note 2 already runs on 4.4.2 based rooted ROM you should only revert back to 4.4.2 stock firmware, same as it goes if you are on 4.3 you may install and stay on 4.3 and then upgrade to 4.4.2 KitKat but do not ever downgrade to 4.1.2 or 4.1.1 due to the current secured bootloader on every update.
I wish to really understand that warning. For example, if the bootloaders are secure, how is it so easy for people to flash custom roms for the first time, coming from those 'secure bootloader' stock firmware versions? What would actually happen and what problems would arise if one did downgrade just as is proscribed in the warning? If there's a "current secured bootloader on every update," why is it safe to flash the same version that one is currently running rooted, but downgrading is not? And so forth…
I'm doing this to several devices, but the best is example is a Samsung Note II currently running Cyanogenmod 12.1, which is 5.1.1 Lollipop. In that device's case, every stock firmware is an older Android version than Lollipop, since no brand pushed OTA updates beyond 4.4 KitKat. If I go back to stock 4.4.2 KitKat, I'm going 5.1 to 4.4—doing what they're warning against. How do warnings like that pertain to devices in this state, and what should I expect?