fyi, this is the worst way to handle a large site migration like this. well, tbh, the worst way would be to completely remove the old forum altogether. as you noted, you've built up many external links to the discussions over the years. i've seen people straight delete the original pages without considering their backlinks so i give you credit for noticing.
anyway, as the new forum has been loaded with all past posts, keeping the old forum alive as an archive is a bad idea. by keeping both live, you're introducing duplicate content to the search engines, making them fight one another for relevance. (google "keyword cannibalization" for a rough idea). this will essentially splinter the link juice and authority you've built over the years b/c search engines don't know which is preferred.
it also harms your crawlability, which is extremely important when introducing thousands of new pages at once like you've done with the new forum.
the optimal way to handle this migration is to setup proper redirects from the old urls to their new versions. it doesn't look like you have google analytics setup on either forum for some reason (no idea why not, please do that), which would be one of the easier ways to build your redirect list. if your developer can export all forum urls from both forums, it'd be very easy to build. if there's no export, using a spider like screaming frog can accomplish both.
the actual technical details on how to achieve this are extremely simple. matching old thread > new thread is a bit trickier since your developer didn't force a unique identifier between them, such as ensuring the thread id in the old url remained constant over here. no biggie, though. ok, i spoke too soon. while the urls are still different, each thread links the original up top. this means the map is already available (linking back to the archive in each thread is also bad, btw, for the same reasons listed above re: duplicate content).
since you have the map linking old > new, just setup the redirects. once the redirects are implemented, users who land on this page will automatically be forwarded here. this tells google that the old url is no longer active and should be replaced with the new one. all previous links to the old pages will forward to the new, too, carrying their link power with it. and since the old urls are already indexed and established, the new versions will replace them quickly. this is necessary to avoid messing up your crawl rates or rankings.
tl:dr: while the new forum is beautiful (delayed registering to old forever due to awful mobile experience), your site migration is done incorrectly. it should be fixed immediately as you've introduced thousands of pages of duplicate content (this is bad). fortunately, it's clear your developer has a map listing old > new urls, which makes setting up the proper migration a breeze. you spent all this time building a new, user-friendly forum, complete with the entire nexus post history (thank god). why go all this way just to shit the bed on migrating fully?
lmk if you need any help or want me to link tutorials for your dev to set this up. i promise, it's very important and very necessary and time is of the essence. sorry for the length. take care.
