My experience (from 17.06.4 to 17.08.1.1, on a 'phone with encrypted data and Xposed, which AFAIK means I have to manually upgrade using TWRP) was mixed, but also illuminating. In brief:
- Used these excellent instructions and at least suffered no boot loops.
- Lost all contact and calendar data. Fortunately it was all remote via DAVDroid, so not too difficult to restore. I had no local contact or calendar data but expect I’d’ve lost those too, given the copious other stories of same.
- Don’t trust Titanium Backup since its “backup” of DAVDroid didn’t include any of DAVDroid’s configuration information (remote calendar and contact accounts).
- As others have discovered, despite the fact that you use TWRP to manually install the update, you surprisingly don’t use the “manual” userdebug .zip file (the “manual” .zip will cause TWRP to give an “invalid zip file format” error): you use the “ota” userdebug .zip file instead.
- I found upgrading TWRP (to 3.1.1-0 using the Official TWRP App) well worth doing because it’s easier to use, and can back up encrypted data partitions (extremely important since Titanium seems to be unreliable).
- I learned that the encryption under FPOOS defaults to something completely bogus: despite enabling encryption on my data partition, TWRP could decrypt it using a default password, without any help from me.
As far as the encryption goes, some very shallow research suggests that it’s possible to go through the encryption process and somehow use a default password without realising you’re doing so, rendering the encryption completely pointless. I’d always assumed that entering my PIN unlocked my data partition, but it seems not. I feel like a complete idiot of course, and probably am a complete idiot, but if I were the FPOOS developers I would have put up a screaming red flashing are-you-sure are-you-really-sure are-you-really-really-sure three page warning before allowing a user to “encrypt” their 'phone using the password (quite literally) “default_password”. Next job is to find out how to correct this.