Having a completely new system sounds like a decent idea, but given that the site is over two decades old, I think it would be good to save an archive of the existing site as there is a wealth of information and discussion here - in fact I often search out old topics on certain subjects. Its also a good indicator of how things have changed over time.
I first started using web forums in the early 2000s when they were quite a new thing, and no one really imagined that they would still be lurking around two decades later. Another forum I'm on lost everything pre-2006 as the forum was hosted by Invisionfree who screwed up somewhere and we were unable to bring the database with us to a new host. I also used to be on the 68kMLA forums - they kept their old Snitz forum as a read only archive which is still usable to this day, but a lot of the subsequent phpBB forum (2003-2007) got completely lost in a big server crash in 2007.