The IRS changed the underpayment penalty calculation and now it looks like I no longer owe a penalty. However, I've already filed. How do I get that portion back?

if Turbotax changes your return afte you file you can hope that IRS corrects the error for you.

If not, you'll have to file an amended tax return.

Always print a copy of your completed tax return for your records, and review it carefully before e-Filing.

You will need it if you are audited by the IRS, or to amend because a TurboTax update changes your return.