As things are, I'd highly suggest urgently releasing an update that would have the tag support feature disabled by default. A per-account toggling on/off this feature might also be worth considering (e.g. off for existing accounts, on for new ones).
From the very short time I had to experiment with v23 before having to revert, the second feature advertised (Improved the IMAP server's performance when opening large mail folders.) seemed like an understatement - with large mailboxes, accessing files was tediously slow pre 23, with 23 it was almost instant. Not being able to upgrade to v23+ to avoid losing existing tags means we will be missing out all current and future fixes/improvements/features.
Regarding the methods suggested, in our case they won't work - too many accounts, folders/public folders for manual processing.
Additionally, we are using a custom post-processing engine that replaces duplicate mails with hardlinks (intercepting queues,hasing and placing to inboxes of group members links rather than actual mail copies). The approaches suggested involve copying the mails - linked files would be replaced with actual files, which, at the next periodic scan, the engine would replace with links again, leading to losing not only tags, but also the read stage of each message.
I can't see any way to really address this issue that doesn't involve (a significant amount of) coding. Building a thunderbird addon as mentioned on the original reply, or an msf parser that would then recreate the imapx.mrk files (if not mistaken, this is where tags are stored) would be too much work for such a specialized one/off task.
After sharing these thoughts, -as usually is the case- I'm returning to the initial suggestion:
Please release a version that has tag support off by default - ideally off for existing accounts, and on for new ones. Such an option would also work well with something similar to what you have suggested : create a new account, copy everything from old to new (tags should be copied as well, right?) then either remove the old account and rename the new, or -even simpler- toggle the tags feature on for the now empty account, move everything back, and finally delete the new/temp account.