The easy solution is to choose someone other than matrix.org, get everyone over there, and hope it works out in the long run

The technical answer is that if you own your domain name you can migrate from a managed solution to a selfhosted one with some caveats. If you can’t migrate the database, then some data will be lost (namely, unfederated rooms and local-only data) and your friends will likely need to do a few things (reset their passwords, and export/reimport their encryption keys). Unfortunately there are no database migrator between different server software right now