Just add the upgrade membership block to the page users are being redirected after a sign up (best using a custom redirect page after signup where you can place the membership block)... And in person/ord mod in studio, you should set one of the roles as default.

Also in the custom redirect page, the can add a html/raw block, telling user to choose membership type or otherwise continue to use default role.

Though if you're using una 13v a1 or a2, there's an issue with custom redirect, which is already fixed and scheduled for next version release. Nevertheless, it works fine for una 12v