The membership upgrade works very similar to how the membership renewal process works.
When a logged in member makes a payment for a membership level, the user will be upgraded to the level he just paid for.
An existing member can log into your site at any time and pay for any membership level. His account will be updated/upgraded to that level after the payment and then he will be able to access the content allowed under that membership access level.
How to Handle the Membership Upgrade
Step 1) Create a WordPress page called something like “Membership Upgrade”.
Step 2) Put the membership payment buttons on this page. You can use the same buttons that you are using on your “Join Us” page or you can create new buttons for the upgrade.
If you want to offer a discounted price for the upgrade then create new payment buttons with the discount price then put them on this newly created page.
Step 3) Optionally, protect this page using simple membership so ONLY logged-in members can view this page and take advantage of the discounted upgrade options.
Handling Upgrade When Using Subscription Type Payments
If you are using subscription/recurring type payments for memberships then the existing subscription payment contract will need to be cancelled. The member can cancel a subscription by contacting you or by logging into their PayPal account and cancelling the subscription from there. It doesn’t matter when the existing subscription contract is cancelled (it can be cancelled before or after the upgrade to the new level).