This will depend on #1142838: Schema needs to be installed when upgrading from 6.x-1.x.

There should be an update function that handles data migration from the latest 6.x-1.x version to the 7.x-2.x version.

I haven't looked at all into this, but given that 6.x-1.x doesn't implement a schema, it would probably involve:

  1. Going through 6.x variables and porting them to 7.x.
  2. Deleting all remaining 6.x variables.