User blog comment:MisterWoodhouse/Updates to Discussions and the plan for Forums/@comment-5558012-20200812173658/@comment-24006128-20200812184529

Yes, Boards transfer into Categories, but cannot merge with existing ones of the same name. If an overlapping name exists, an additional category of the same name will be created.

Admin and mod teams would need to move threads from one category to the other and then remove the duplicate. It's an unfortunate limitation.