We're switching to Bugzilla version 2.17.6, the one that's also used on the largest and best known bugzilla site. Although not technically a stable release, it's very solid and its branch has been used and tested for a long time (the "stable" Bugzilla branch, on the other hand, has been rather dormant and stale over the last two years). I don't expect any data integrity problems whatsoever, but we'll keep a backup copy of the Zilla database at the moment prior to switchover, just in case any unexpected failures turn up.
As is the case with current Zilla's look, the new Zilla will be templated to look vaguely like LiveJournal; this cannot happen automatically (because templates changed too much between versions) and may not happen at once, so don't be alarmed if you see the default Bugzilla non-LJ-like design at zilla.livejournal.org — it'll be customized over the next few days if I can't do it all tonight (I just don't want to stall the upgrade any more, it's been long overdue).
By the way, in case you don't know what I'm talking about, please meet Zilla, our friendly LiveJournal bug-and-enhacement tracking database. Zilla helps us track bugs and hunt them down. It also helps us keep track of small enhancements as well as new exciting features we want to develop for LJ. Zilla is good for you. We love Zilla. Please use Zilla! This concludes our annual please-use-Zilla advertisement.