You don't even need a DAR. You just notify the FAA that you want to put the aircraft back into phase 1. Typically they'll say 5 hours, and when you've flown the 5 hours after making whatever change you want, you simply make a note in the logbook and you're done. This assumes you have the newer format operating limitations, that say a major change requires new phase 1 testing. Older op limitations say that any major change invalidates the airworthiness certificate; in that case you need to apply for new op limitations first.

Changing the MTOW would be considered a major change. But note: if the MTOW was ever recorded at over 1320 lbs., it will never be eligible for operation by a sport pilot, even if you lower the MTOW to 1320 or less.