Keeping your existing transaction history is important. If you already usy a different app, you probably want to migrate it over into Actual.
Right now, only YNAB4 is officially supported. However, the API allows anyone to write a custom importer. We will work with the community to help write other importers soon.
YNAB4 is the only builtin importer right now. We're hoping the community can help build other importers on top of the API.