My recommendation is to wait for the amendment... you don't have to pay any additional tax due until April 15th, 2019 anyway since penalties and interest start after the due date.
Regardless how small of a change you need to make, or whether your form can be sent with or without the F1040X (the amended return), the only way to make a change to the original return in turbo tax is through the amendment process. It's because the recalculation is done on the original side of your return using the original data entry.
If you received the IRS letter and have a deadline, please call them and ask to extend it, if the form is due before TurboTax releases the amendment. They are usually fine doing it. The amendment is tentatively scheduled for release on March 7th but again, you'll have no penalties or interest if you pay any additional tax due on or before April 15th.
Your other option is to print the F8962 and its instructions and make the changes manually, but be careful of potential errors...
