If you read C:\AVEVA\Plant\PDMS12.0.SP6\Manuals\Docs\PDMS User Bulletin12.0.pdf Chapter 23.7 Incompatible PML changes, you will see that some in house customisation has to be checked and modified to be 12 compliant.
HTH
If you read C:\AVEVA\Plant\PDMS12.0.SP6\Manuals\Docs\PDMS User Bulletin12.0.pdf Chapter 23.7 Incompatible PML changes, you will see that some in house customisation has to be checked and modified to be 12 compliant.
HTH