Shouldn't be a problem going to Watertown as According to the MassRMV website you do not need an appointment for registration transactions.

https://rmvmassdotappt.cxmflow.com/Appointment/Index/2c052fc7-571f-4b76-9790-7e91f103c408

Registration Related Transactions

Transactions that CAN be performed at an RMV Service Center without an appointment:

Register My Vehicle Reinstate My Registration Transfer Vehicle to a Surviving Spouse Amend My Registration Confirm Identity for a Registration Renewal

Just make sure you have your stamped RTA, bill of sale, and payment including taxes.