Anyone have thoughts for pitching a migration of rulesets for an established tabletop game?

I've got buy in in terms of new system and moving from 5e to a PbtA system, but they're all suddenly talking about building out entirely new characters because the playbooks don't match 1 for 1 to their previous classes/characters.

Any tips for massaging that transition if I'm invested in keeping the same characters involved in the story?