As per the title.
If I was running a Vanilla forum and wanted to migrate over to a new lemmy instance, e.g. forum.com to lemmy.forum.com Could I migrate the entire forum over while keeping the same categories/sub categories as communities I guess, and bringing all of the old threads with them? If it is possible, would it be difficult?
Not that I know of. Thats the case most times a community has to move, you more or less start from scratch.
You can set up a bot to periodically post links to old threads, articles, etc but that’s about it afaik
Seconding the suggestion to start fresh. Make the old forum an archive, still readable but impossible to add or edit. If necessary, migrate it to a more lightweight system, perhaps one designed specifically for preserving old forum content (I don’t know if there is one, just spitballing here).
You really should give some thought to whether Lemmy is a good format for your community. Forums are very different from Lemmy.