The only way to do it is the way you suggested. Create a new Premier login and have the forums switched over. Remember, only forums transfer. Realms, access groups and other resources do not transfer.
To be honest, the only time it matters is when the forum owner posts. If you want to save a ton of work, just keep the forum ownership as-is and use the owner login only to maintain the forum. If you like seeing the "Forum Owner" tag on your posts, create a group named "Forum Owner" or something similar and put your other login in that group.
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