The law is actually clear that taxpayers would be entitled to the difference between the advance refund  (EIP) and the Recovery Rebate on the TY2020 return but excess, if any, will not need to be repaid.  This means that the parent will compute the Recovery Rebate without the newly independent "child" on the 2020 return - if they are entitled to a larger Recovery Rebate than the EIP received, they get to claim that as a credit on the return but they do not owe the IRS any excess if the EIP was larger.