Instances have a different meaning in Proe (e.g. instances of a screw in an assembly or number of times the same screw is used in an assembly).
What you refer to as instance are the "Version Number". This can be a problem without a PDM system. What you can do is to create a Shrinkwrap and replace the assembly with the shrink wrap in the GA. The GA itself can contain "Graphic rep" of assemblies. [one cannot edit Graphic Rep entities].
At the end of the day, you can "Purge" the files to remove any previous saved versions. Thus, the person who makes the GA will always get the shrinkwrap/Graphic rep of the latest version.