If you both agree to
file jointly, then you'll have to amend your husband's return.
The IRS can't have
the original MFS return hanging out there in your husband's name, then have an
MFJ return come in on your name. They'd
have to contact you both, and you'd spend weeks (at least) sorting out the mess.
Please see the
TurboTax FAQ "How to amend (change or correct) a return you already
filed," below, for instruction on how to amend your husband's previously-filed tax
return.
https://ttlc.intuit.com/replies/3288565
