Use:

WHEN MATCHED AND '+REPLACE(REPLACE(C.UpdateColumns,'=',''),',',' OR ')+'

Result:

Tgt.[DateID]Src.[DateID] OR Tgt.[ProductID]Src.[ProductID] OR ...

I think here the sign '<>' is omitted?

h.e. Tgt.[DateID]<>Src.[DateID] OR Tgt.[ProductID]<>Src.[ProductID] OR ...