Two-step subscription; how to allow users to sign up for only one subscription at a time

I'm looking to create a two-step subscription process. I would like to allow any user to sign up for the first tier of membership, at which point an admin would review their submission. If the admin approved, they would enable the user to sign up for the higher level of membership.

The workflow would be: 1) New user completes form. 2) WordPress admin reviews and approves submission. 3) New user is directed to signup for higher membership tier. 4) New user gains access to members-only content.