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?
If you don’t care about the migrated content being federated, then you could write a script which copies everything into the database of your Lemmy instance
If you do care, then you would have to go with the other person’s suggestion of having a bot post the migrated content periodically
If the forum users are local, there’s no issue federating them. That is, if the db was migrated in the first place.