When you enable mediation, a new publisher ID is generated for you by AdMob that you should use in your app update. Banners that use this new publisher id will get the mediated banners. Your older apps, that still use the old publisher ID will continue to only serve AdMob ads.
I understand.
That means I can enable mediation before publishing the new version, test with the new ID and once everything works, simply publish the new version?
Absolutely correct - enabling mediation (or “creating a mediation placement”) will not affect your old publisher ID at all. Users of the old version will continue to receive ads as normal.
Mediation IDs and Publisher IDs are treated as completely separate things in AdMob. So you can edit them independently. You could even create many mediation IDs for the same app if you like.
If you go into the mediation settings, you can choose which Publisher ID it should load ads from. This is configured in the same place you setup the third-party networks for mediation.