My Model Data area library is Y2SY. So, I restored Y2SYVENG into Y2SY. I didn't change
National language as we use default *ENG.
When I first ran the upgrade utility YAPYMDLCHG it didn't work and gave the following error
"Object Y2SYVENG in library QSYS not found." I restored Y2SYVENG into Y2SY, and Y2SY was in the library list hence I shouldn't get this error.
As you have merged Y2SYVENG into Y2SY, then you need to update *DTAARA YLNGENGSYA to Y2SY. This indicates where the English language library is located (needed to update YLNGENGSYA from Y2SYVENG to Y2SY). After that YAPYMDLCHG should work.