Checkout when selling memberships

I sell memberships to my site using Pricing Plans. The standard Wix user interface for the checkout is fine with me except for one big problem. Users who are already members see the interface and press the “Sign Up” button because they don’t read carefully enough. The result is several users who create duplicate memberships. I’d be happy if I could simply switch the order of the two buttons. Is there a way to do this without doing a completely custom purchase process?