[Fire TV Q&A, Help & Troubleshooting] thatthingisentyou:
TLDR : If you have a registered running rooted prior to Fire OS 5 version, back it up! before you attempt any upgrades, if you have to wipe a Fire OS 5 install you'll be in trouble or at least have a lot of extra work, because you can't init/register without a connection, which would likely mean you lose root.

I've been staying on the latest pre-Fire OS 5 for a while because there was no reason to upgrade. I didn't want Alexa, etc.

I started running into the mbcache.db issue and I cleared that file which resolved the issue, but at about the same time the amazon content stopped showing up with unable to connect messages. Unsure if they are related as yet.

Eventually I wanted to watch some Prime shows so I decided to upgrade to an early version of 5, thinking it might resolve the issue. It did but I saw an updating message and I panicked thinking my update blocks had failed. I later figured out it was the remote that was updating, which is a concern in itself but that wasn't before I ran into difficulty.

(At this point I will point out it seems the likely reason for a remote update is so it can listen at all times, but that may not be of concern to some people.)

So thinking it had started a box F/W update I did a factory reset from TWRP and rebooted. The first thing I will note is the new boot menu selector looks very 'official' and I didn't know it would change from the old colorful RBOX boot menu. This freaked me out right away. I didn't see anywhere this would change.

Since I had done a factory reset wipe I had to go through initialization and registration. This no longer can be done without a network connection in Fire OS 5. Be AWARE! If it can be done I wasn't able to figure it out. Previous guidance about when to unplug and when to plug didn't work. It will just hang waiting for a connection.

So I had to revert to a the previous non 5 version. I did, but the new, less kitschy, less RBOX menu screen was retained. I didn't like that and wasn't sure if it was a problem, so I reinstalled the original RBOX boot menu. That may not have been necessary.

Since I had done a wipe I did have to reregister but this time it did work without a network connection using the old tried and true ways.

At this point I was back running, but Amazon content didn't show up. Sort of confirming that pre 5 OS will not load Amazon content. (Please disconfirm if you know that it CAN run. I'll probably go back if so.)

I had the sense to use TWRP to back up the 4 relevant partitions at this point, to save myself some trouble if I need to revert again.

Once I was registered under the old version, I was able to upgrade again but this time with a current stored registration. So it didn't need to initialize or install in Fire OS 5. This worked ok.

I didn't lose root, so I should be able to turn off Alexa.

Since my remote has been updated, I'm not sure I even could go back to before Fire OS5. If anyone has info regarding that please share.

Probably the smart thing to do would be to install the first TWRP version of the OS and back that up for safe keeping. Since it's working now I probably won't follow my own advice.