It's easy enough to have multiple microSD cards. I buy labels of two sizes and mark mine. So I can play with the Py2 rig to do a print, finish that, shut it down, swap in the microSD for the Py3 work and continue from there.
The Python 2 countdown clock says...
The reality is that we should start seeing a cascade of fail, basically. You might try to update a plugin on your Python 2.7 rig and one of its dependencies fires off and fails because that author no longer supports Py2 for, say, the RFID authentication stuff, for example. So the first line of fail is the dependencies which plugin authors here really can't control that well.
If you have more than one required third-party plugin then you might want to try camping out on Py2 for as long as you can stand the pain.
But if you're like me, you'll just wave off any non-compatible third-party OctoPrint plugins, setup for Py3 and drive on. I'll eventually get those back in if I truly need them in the future when they upgrade. Or perhaps I'll write my own if I need it that badly.