Old sessions will load with old audio connections. Reload your newest audio connection preset in those projects (create a preset if you don’t have already) and save the project, then they should open with the new I/O.
PS: Every audio connection tab has its own preset.
Nope there is no way to do that, if the plugin manufacturer did not code the new version to replace the old one. If I am not mistaken its an inbuilt Plugin ID hardcoded into the Plugin dll/vst file. There is nothing Cubase can do, if its a different ID.
If you still have a dongle you can run older versions of Cubase that will work with the old VSTi. Render that to Audio and import that Audio into Cubase12.