Updating custom forms / Customer records

With version 11, we're seen some layout variations in the customer record screen.  We went back and modified the default customer form to capture some of the new features ('sales tab' for companies instead of just 'transactions') etc, and saved this as yet another custom form and made it default.  But now we seem to have a mix of forms in use; the old customers appear to be using the form that was the default when they were created.

It is possible to assign the new custom form as the default form for all EXISTING customers?  (Alternatively, I'd hoped for a mass update feature but haven't spotted it yet.)