From the www,mass.gov website (emphasis added): "If you're changing from married filing joint to married filing separately, you and your spouse may file separate returns before the due date of the original return even if you already filed a joint return for the tax year." Here is a link to the instructions to Amend My Return in TurboTax.
For next year - and maybe this will help with your amend process - try filing with the "mock" Federal method outlined in the TurboTax FAQ below.
https://ttlc.intuit.com/replies/3301995
