Thank you that confirms my suspicion. I do have a twist in this case. My clients have a daughter who was named in the coverage. However, in 2020 she got a job and was no longer a dependent. To optimize the refund, I gave the parents 0, and 1 to the daughter  as she had the lower wages. 

So the parents are already at zero. The daughter was 24 years old. In this case, I don't think I need to amend the parents return, but do I need to amend the return for the daughter who got the benefit of the premium credit.

Please advise