You can install ardour v6 and v7 running in parallel, in fact as many different versions as you like.
When a new major version of ardour is started the first time, it detects that fact and asks if it should copy over the config folder from a previous version.
When trying to open a session created with a different version (even Mixbus), ardour (any version) creates a backup file and tells you in a dialog how it got named, and to use that when opening it with a previous ardour version in the future.
Opening a newer session format with an older ardour version is calling for trouble.
Apart from this backup copy and writing to the state file (instant.xml) ardour doesn’t alter the session unless you record audio or explicitely save it (as far as I know).
All that said, I normally seamlessly continue working on my files with new major versions, but I’m not too cautious as I rarely work on really important stuff these days. If I do, I’ll avoid updating ardour, just to be safe and don’t lose time with fixing installations etc.