Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
Creating a new den and transferring them in from that new den’s Roster is the best way to proceed. Don’t try ending the memberships one at a time. That will just confuse things, and potentially drop scouts from your Scoutbook roster.