After the upgrade we found ourselves in this situation where all the FI documents (that you can display in FB03) have the fields XREF1, XREF2 and XREF3 greyed out. We can no longer change the contents of these fields in FB02. Unfortunately, we missed this in our testing phase and we are in production now.
I verified SAP notes and could not find any relevant information. I also verified the configuration settings and did not find any issues.