Membership: Upgrade to paid from free trial not available

I have a 7 day free trial set up for Visitors. Until this morning, a free trial user could upgrade at any time to a paid membership by clicking "Upgrade" on a paid membership. But this morning I am getting a "upgrades not available" button on my paid memberships.

How do I change this so a trial user can upgrade to paid at any time?

This is a brand new site just about to go live, so any help/ advice is greatly appreciated!

Michael