I tried the update like written in the subject and experienced two issues. Both issues break the update and leave the database in a state which is not working for the old version.
1. the update script printed out "Database error in line 452: Specified key was too long; max key length is 767 bytes" and the update did not finish.
I noticed that this issue was discussed already, and I also did some googeling. Probably this isuue is somehow related to an old SQL version, in my case it is "Server version: 5.5.62 MySQL Community Server (GPL)", i.e. I agree with the explanation from user Auge.
I did not really understand if there is already a bugfix. If yes, it looks like it did not work properly. Maybe the best solution is a SQL update.
2. just for fun i tried a quick-and-dirty hack (not recommended for others). I changed in the update script line 448 "VARCHAR(256)" to "VARCHAR(255)", because 3 * 255 < 768. And indeed, this error was gone, but another appeared: "Database error in line 452: Duplicate entry 'firstname.lastname@example.org' for key 'key_user_email'". It turned out that there are two different usernames with the same email address.
Thats a case of a bad administration, in the first place I will remove the double entry.
But I think it should checked before updating or at least mentioned in the update hints.
I installed the backup of my current forum, so there is no damage and I can prepare the next try without stress. While installing the database backup I noticed the new version added four tables. i.e. PREFIX-akismet_rating, PREFIX-b8_rating, PREFIX-b8_wordlist, and PREFIX-uploads.
It turned out that these tables remained in the database after installing the backup and caused errors at the next try.
I would suggest to offer some short fallback hints in the update description, maybe:
"Before update: back up database and php/html code.
If it did not work:
replace code with your backup
replace database with your backup
delete the newly added tables, if still present"
have a nice day!