It may be slightly too complicated to actually change the post ID because it's likely to be linked in bb_posts, bb_posts_text, bb_search_wordlist, bb_search_wordmatch. I'm not 100% sure if this is the case but if so, you may want to try something else such as:

...if your certain about the user ID that was deleted, simply create a new user in your admin panel, then go into the db and change the user ID# number to match the user you deleted.

or,

...if you really need the thread why not simply copy if from the database and re-post it in the forums if your really stuck?