Export your client list from the Trainerize Clients tab, drop it into the Marrow importer, check what will land, and invite your clients. They enter their card once, at your price. Trainerize exports five columns, so workout history and programs stay behind.
Invite each client right after their last charge on Trainerize. They are charged on Marrow the day they join, so there is no free gap and no double month. Or, on a monthly price, set each client's first charge date to their old billing date, 3 to 45 days out, and invite everyone at once: they add a card with nothing due today, and Marrow charges them on that date. As each client joins, cancel them on Trainerize, one by one. The importer lists who has joined, with a Done tick. Never cancel anyone on Trainerize first.