Allowing extensions to run after a board updates to 3.2 has some potential pitfalls. Since phpBB will be updated before any extensions can be updated, and certainly some extensions simply won't be ready for 3.2 for a while or even ever, possibly, we should disable them all during the update. This is simply to insure that some extension that may have code that could cause an unforeseen fatal error to phpBB will be avoided, thus saving the support team and the admin's a lot of grief.
Users would then manually re-enable their extensions as they test out their new board, and by that process they might able to discover if an extension is problematic.
Another potential issue relates to this ticket: https://tracker.phpbb.com/browse/PHPBB3-13802
In this scenario, a board updating to phpBB 3.2 with the ABBC3 plugin installed could result in the loss of BBCodes, due to the reparser applying ABBC3's bbcode permission settings to the user during the reparse. Any BBCodes broken this way will not be repairable by subsequent uses of the reparser. And even if the ticket produced a fix that would allow ABBC3 to not influence the reparser, there is no guarantee people updating their board to 3.2 would have this updated version of ABBC3, leaving them still vulnerable to ruining their posts.
So I propose simply using a migration that sets all installed extensions to disabled state, and make it a dependency of the text reparser migration.