Which PO fields cannot be changed by SAP rule?

Hello experts, I have a doubt about SAP functionality.

Which fields cannot be changed by the erp rules?

In my company, we have replaced the company code ABCD to new company ABCX, and we have a lot of PO non-productive that has not been received. We are planning to change the company code, plant and cost center on the POs so it can be used with this new company ABCX..

The POs are being created (and will changed) thru BAPI.