A member cannot belong to more than 1 membership. Instead, you can setup so each membership has different content.
Then make the membership that the member upgrades to include all the content from the member's original membership.
You can do this by setting up content groups and attaching them to content.
If an existing Member purchased into a separate membership if they use the same email, then they would cancel their old subscription and would belong to the new membership. If they use a new email and have 2 separate accounts, then they would have 2 separate logins and see separate things in each members area.
A. Direct Payment - Add SolidTrust Pay button:
1- login to your solidtrust pay account
2- Go to my account -> view profile page
3- copy your username
3- Go to Merchant Zone -> Create Subscriptions
4- Click MEMBERS - view the items you are subscribed to
5- Copy your subscriotion Id
6- Paste both username and ID to your payment processors page