The same as any other return. If you renumbered the client, the proforma that client number. It is done in the 2020 return, Tools > Tranfer Clients (Proforma to 2021)

Not sure I understand.  Since I added an X to the client number to amend the 2020 return, I transfer this to the 2021 return and remove the X?  

X marks the spot ———- and it doesn’t hurt leaving it in the client number.  Just don’t transfer the original return too ———- that could lead to confusion down the road.  That’s  why I never make a copy - I just correct the original return.