Membership plan of any member can be updated by the Group Admin/ Moderator.
Search the member in Active or Expired Memberships list or do a top level Search, click on the "Action" icon next to the particular membership. Click on the Membership Tab and then click on the manage icon. You will see a drop-down with "Change Membership Plan". Click on it and you will see all the plan for your group / Chapter. Select the plan and save it.
This call to action is being used to upgrade or Downgrade the membership plan of a member.