Re: Changing Topic ID
You can change the autoincrement value for a field in phpMyAdmin. If you’re modifying the ID you will be using something like phpMyAdmin. I think there is a top menu item “Operations” then on the next page find the “table options” section and find the auto-increment value and just change it to whatever you want your next one to be.
For changing the existing topic_ids, I think there is also a reference to topic_id in the _posts table (if not, how would bbPress know what topic a post belonged to?) I think it also might be referenced for tags too. I didn’t check, but I think tags are attached to topics, so the tag would have to reference the topic id somehow as well.
This is all just thinking out loud. You should probably back up the database first then make your changes and see what does not work. Might want to recount afterward.