If a member wishes to upgrade/downgrade their account then they should first go to /admin/billing and cancel their account there. Then they should login again and on the suspended page click the option they want to upgrade/downgrade to.
They can also order again from the order page as long as the email they make payment with from the order page matches the email of their account it will reactivate them. It may take them to the join page where they fill out the form to create a membership, but they do not need to fill out the form because they already have an account.
This answer does not apply if you are using direct payment to members.