Question   How can we upgrade Members to another membership when they send us payment manually?

A. When editing the membership, in the URL at top you can see the id for the membership.  Then, use that id for the membership and add it to the url as shown below replacing the id number with ID:
/join/USERNAME?membership=ID

So, for example if it is for username john and membership id 3 it would look like:
/join/john?membership=3

Or if putting it inside the WYSIWYG of a capture page you would put the following code for membership id 3
/join/~username~?membership=3

A. Because you are selling multiple memberships you would need the $600 Tool Truck.

A. 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. 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.

A. 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.