Several users (myself included) have made the unfortunate mistake of attempting to downgrade their bootstack (for various reasons) to a former version.
DO NOT DO THIS (at least for the USA T-Mobile H811 version, and possibly other USA versions, per @autoprime)! It will result in a hard brick! Specifically, this results in the "Qualcomm QDLoader HS-USB port ( 9008 )" error. Connecting your phone to a PC will pop up with this active connection, but nothing else. There is no download mode. There is no publicly available fix as of yet.
This WILL happen both through KDZ restore as well as flashing bootstack .zips.
Yes, an official KDZ image can BRICK your USA phone, if you try to rollback.
This is in stark contrast to other manufacturers (i.e. Samsung), where the attempt to flash an older bootloader/modem/radio will simply fail, but your phone is not bricked.
I have conferred with @autoprime, who confirmed that with each OTA firmware release (on T-Mobile it was 10H, and now 10N), the version # increments. Flashing a lower version # will result in brick.
I have searched far and wide for a DIY fix. Without the proper firmwares in the right format, as well as the right flashing software, your phone is done and must be sent in for LG repair.
To check what antiroll-back version you are on, input this sequence in the dialer (T-Mobile and ATT) and scroll down:
Flashing a bootstack version less than this # (through KDZ or zip) will brick the phone. You are free to flash anything equal or higher than the version listed.
The 20i MM update for the H811 is v2 antirollback. The 10n bootstack is also v2. So flashing between the 2 (returning from MM to Lollipop) should not hard brick the phone.
The 20o MM update is v3 antirollback. Once you go 20o, you cannot go back.
For your flashing needs, I suggest you use LGUP and NOT LGFLASH, as LGFLASH will ignore the rollback flag and brick your phone without remorse!