When to amend 2018 Fed and State returns (which have been accepted) due to incorrectly labeling an RMD from a beneficiary IRA? The correction will lower both refunds.

Our Federal and State (CA) returns have been accepted, but refunds not yet received.  Although I did enter code 4 "death", in box 7 when entering the 1099-R for the RMD from this beneficiary IRA, I missed the question in the interview which asked whether the distribution was inherited from someone.  So the RMD from the beneficiary IRA showed up incorrectly on form 8606.  Now that I've corrected this, our refunds (both Federal and State) will be lower by several hundred dollars.  I'd love to correct this before getting the refunds, if possible.  But what's the best way to proceed?