To be honest I’m not sure what I expected, in fact if you had the cubase 7 trial you didn’t even need to install cubase artist 7, just adding the license to your USB key would allow you to use that install as Cubase artist 7.
Either way, uninstalling cubase 7 shouldn’t harm cubase 6 at all.
I’m not entirely sure what the most efficient method is for getting all your plugins in Cubase 7. For some it probably works if you just copy the .dll files from the VST plugins folder for Cubase 6 to the Cubase 7 folder. I don’t actually know how to go about VST3 plugins, I doubt you need to install them again but I don’t know how to have them show up. Someone else maybe?