Promotion Group and Upgrade / Migration Functionality

Hi Gurus,

I would like to understand the behavior associated with upgrade functionality for Promotion Group. I checked the TOIs as well as Tech support and while I could gather details on Modify and Disconnect behavior with regards to Promotion Groups, i don't understand the OOTB intended behavior for Upgrade

If for instance I have two Promotion Groups say Test PG1, Test PG2 with the following structure:

Test PG1

  - Membership 1 (Min 1, Max 1)

      - Product 1 (Min 1, Max 1)

      - Product 11 (Min 0, Max 1)

  - Membership 2 (Min 0, Max 1)

      - Product 2 (Min 1, Max 1)

  - Reward 1

Test PG2

  - Membership 1 (Min 1, Max 1)

      - Product 1 (Min 1, Max 1)