Since upgrading, my users are reporting they cannot edit their own posts in forums if it isn't their latest post in the forum.

I recommend setting a time limit for editing own posts to prevent self destruction of useful information. I've seen good people lose their composure and wipe out incredibly helpful collections of posts.

Edit Time Limit (minutes): This sets the length of time a message can be edited before message editing is disabled.

That's already part of the software, the option above allows users to edit ANY post, not just the last.