Yes, keeping the re-direct scripts in place
You'll need to update to ubb.classic 6.7.3 prior to importing into threads.
Edit: On second thought - your links to other topics in the 5-series UBB go to xxxxxxxxx.html files - if you want people to be able to find those you'll need to leave those .html files in place. Once someone clicks on another link within the ubb the re-direct scripts will forward to the current location of the the forum, not to an individual topic.
It's possible to do a search/ replace to update the topic-links within the posts, but that would need some extra thought
