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.
With direct payment turned on, if there are multiple memberships setup, then a drop down appears for the person to select which membership to join. This controls if the membership appears in that drop down.
, people can purchase external billing products without being a member.
It creates a contact for the record of the client and the sale is attached to the contact.