Jason is correct in that stopping QVS will stop any active user sessions on the AccessPoint, so suggest you make the change during a scheduled server maintenance period. You can check to see if any users are active by looking in QMC > Status > QVS Statistics > QVS@ > Active Users tab before stopping the QVS service.
I would caution that enabling the "PGO as XML' will cause a performance hit as the near constant read/write activity of the PGO files will in effect double, as the XML files will be generated. Qlik Support suggests only using the 'PGO as XML' configuration for short a duration, such as troubleshooting.
Hope this helps,