"If it ain't broken, don't fix it."

1. try to migrate the old forum to a new one using a service like https://gconverters.com/forum-to-forum-migration/
2. make the current forum a legacy one and close it for further posting but open for searching and then start over with a new one.

There's a few problems with these: Do you have any idea what a conversion through gconverters will cost? Not changing costs nothing.
Keeping an old forum next to a new one is about the worst solution there is. Do you have any idea how large the knowledge base is in this forum? Do you always want to search through TWO forums?

IF and WHEN Emmanuel starts looking into new forum software he can contact me for conversion. It's my business and professional specialty. And free is a lot cheaper than gconverters.