If a session is opened with a Pro Tools 12 system on which plug-ins aren't available that are used in that session, PT provides the option to buy and download those missing plug-ins from the Avid store.
But some plug-ins are already on the system, but in a new version. They only need to be exchanged within the session. This shouldn't be a manual task.
For example: If a session containing the VSonics VMeters plug-in 1.2 (RTAS) is loaded to a PT12 system with the 1.3 (AAX64) version, then Pro Tools should not only ask if the user wants to buy it, because it is already available. So an additional option is needed, that lets the user directly chose one of the installed plug-ins to be the one for replacing the old RTAS / TDM plug-in.