There is an option on the Registration Form that allows existing Members to simply login after they have paid to upgrade or gain access to an additional Membership Level.
This can also be done for free membership levels as well.
A user can click on the Payment button to upgrade/purchase additional Membership Level and then sign in using the “Existing Member” login INSTEAD of filling in the Register Form.
If the membership level is free, then the member can just use the “Existing Member” login option on the registration form without needing to pay.
You can create a payment button that allows people to sign up for the Membership Level and IF those people are already members of another membership level, they can use their already existing User Account.
The same applies to a free membership level except that they don’t need to purchase first.
Also, be sure that the “Disable Existing Users Link” option on the membership Level is not checked. If this setting is checked, it removes the option for Existing Members to Login.