The issue is with the Mortgage Interest Form 1098:
BACKGROUND:
My lender did not send me a 1098 form, so I called and requested one. I did ask for the Box 1 and 10 figures but did not ask for Box 2 (stupidly). When filing I thought that the Outstanding Principal Amount was at the end of period instead of the beginning. I just received the Form 1098 and have discovered my error.
No, if correcting the outstanding principal balance does not change your refund or tax-due amounts, you do not need to amend your tax return. TurboTax asks for that amount in Box 2 because the amount of mortgage interest you can deduct is limited. But if your outstanding mortgage balances are not more than $750K, the average mortgage balance calculation is not used. It sounds as if you already started amending to repair the numbers - and already know that correcting the box 2 amount does change your tax position.
Do I really need to amend my return?