T8607 - Have Size 10 - Need Size 12

My daughter bought a Michaelangelo Style T8607, Size 10, from David`s Bridal a few years ago and then decided her fiance at the time wasn`t the man of her dreams after all. She has now found the right man and she still loves the dress but now needs a Size 12. I think a street size 6 = wedding gown size 10 and the street size 8 = wedding gown size 12.
She`d like to sell the Size 10 and buy a Size 12