The Right Approach to Handle Cross Validation Rule Changes

We are making changes to Cross Validation Rule. Specifically disabling some combination of the segments in Chart of Account. However, these combinations are already used in PO, such as PO Charge Account or PO Variance Account. If we enable these rules, system is not allowing posting of payable invoice to these affected POs.

1. What is the right approach to handle these Cross Validation Rule changes? Our concern is even we clean up open PO with these combination, PO can be reopen for returns / credit note later.
2. Another point is that for PO that is already partially invoice, system is not allowing us to change the PO Charge Account or PO Variance Account. Does that mean we have to wait until the PO is fully invoiced before we can enable the new rules to disable the account combination?