I want our members to do an "upgrade", so the "Create Profile" section is not necessary.

If you want existing members to do an upgrade, you should use the Billing Modification Forms. Please see: WordPress Dashboard s2Member (Pro) [Payment Gateway] Pro Forms Billing Modification Forms. If you present that upgrade form to a user after they login, they won't need to "Create a Profile", as all of their information will already be filled in.

What if I don't need membership features at all; if I only want to sell something one-time?

If you don't require the visitor to signup for a membership, but instead only want to give them access to a single Post/Page (i.e., you want to sell access to a specific Post/Page on a one-time basis), you can use the Specific Post/Page (Buy Now) Forms. See: s2Member → [Payment Gateway] Pro-Forms → Specific Post/Page (Buy Now) Forms