Changing Default Gateway in Membership

Hi
How can I change the default gateway from 'admin' to 'PayPalExpress' ?

When a user registers on the site and creates a Free account they're assigned to the 'admin' gateway by default which means they can't upgrade. If I manually assign the user to the PayPalExpress gateway after they've registered then the user can upgrade immediately. So how do I make all new users default to PayPalExpress?

Cheers