Remove Previous Subscription When Upgrading?

I currently have two paid membership levels. I want new users to have the option to sign up for either Level 1 or Level 2. I'd also like existing Members of Level 1 to be able to upgrade to Level 2 at any time. I have this set up, but when they upgrade, they remain a member of Level 1. Is there a way to set it up so that when an existing Level 1 member signs up for Level 2, they are removed from Level 1?