I have the same problem. When new versions of VST’s are installed the old versions are sometimes overwritten or uninstalled (say ver 4 to ver 5). When I go back to older projects they ask for the older versions. Sometimes I can just select the new version and providing the same settings/patch is available there are no problems. However sometimes I need to know the patch name but Cubase does not always display this info. Occasionally I reinstall the old version of the VST then I can get the patch name, and then go back to the new version. This is all very time consuming and does not always work.
So I totally agree with SoundOf, more info would be great.