Unless they fixed it since last year, TurboTax gets this completely wrong and miscalculates your tax returns. But it's OK because there is no way to contact TurboTax to let them know. 8-) All we can do is talk about it here.
Just lie to TT and tell it you're still on the original mortgage.
BTW, it isn't settled law yet what happens if your original grandfathered mortgage was over $750,000 and you then re-fi it and take cash out. The tax lawyer I consulted said that it may end up that you lose the grandfathering by doing cash-out, but more likely that only the original amount is grandfathered. You can't take a $800,000 mortgage pre-2017 and re-fi it into a $1M mortgage and claim the full deduction on $1M.
