I am also facing similar issue migration of the lower ENV rules after upgrade. We changed the ENV to PRPC and Restarted the application server. Can able to access the URL but not able to login with Administrator@pega.com operator too. Only can see Blank Screen. Not able to proceed with the 3 rd step you mentioned above. Can you please help me to understand the step3. Do we need to change the access group in DB or from PRPC portal?
Check the prconfig.xml from Pre-upgrade system and use the same value in <env name="Identification/SystemName"> tag in the new environment prconfig.xml. It is not necessary to have the system name value as prpc always. If it is customized, use the same name in the new environment. Also, verify the logs to see if the error occurring 'Data-Admin-Requestor instance "pega!BATCH" not found'. If yes, changing the SystemName would solve the issue.
I have seen multiple queries/posts with above system name issue in upgrade forum. This seems to be one of the common issue encountered during upgrade. Can we have an article published with the problem statement, root cause and the steps to resolve? We do have multiple articles on common upgrade issues encountered. I am not sure if we have one for the above mentioned issue.