They make it sound damn near impossible to switch a NY registration to MA, and neither state gives an F that their laws conflict with the other’s.

MA requires you to have MA insurance before you resister your car/get your MA plates. NY requires you to surrender your NY plates and registration before you cancel your NY insurance (if you don’t, they will suspend your license). MA does not issue temporary plates, and you must have plates on your car. There is no grace period for any of this. Some people have managed by temporarily holding two car insurance policies. I somehow managed without doing that, but rather switching everything on the same day and mailing the NY plates back the day I got my MA plates.