Can I re-install MacPorts but leave my Python installation, done via MacPorts, alone?
The answer is unknown.

Macports is only supported and tested if you do a complete reinstall of it when you chnage the versions of macOS used.

You might be get away with rebuilding only a few of the ports but if you have a problem the only support is delete and reinstall as no one has the time to see what happens when you chnage the OS.