Just so we have the circumstances correct here:

1. Client filed their own return for 2018 and did NOT include the 1095A and 8962.

2. Client recently got the dreaded IRS letter requesting the 1095A and 8962, sent them in and got the bill for the payback of the APTC.

3. Going through the 2018 return you find an error that if corrected, means they had less of a tax liability in 2018 and changes the payback of APTC.

4. You are preparing a 1040X for 2018 which will correct the error and change the APTC payback...so all forms that changed from the original will need to be included with the 1040X, including the correctly computed 8962.