We added an Event Transfer API: https://test.runsignup.com/API/race/:race_id/switch-participant-events/POST.
This allows our technology partners to make more transparent syncing between their application and RunSignUp.
This works by creating a brand new registration for the registrant and removing the old registrant, so a new registration id is generated. Also, only the basic information is copied over to the new registration. Things like shirt sizes, question responses, team names, etc. are not copied.