Technically illegal? That is completely incorrect.
The FCC mandate that requires all NEW activations be on AGPS enabled equipment specifically addresses current subscriptions on non-AGPS equipment. Users with non-AGPS equipment with current subscriptions are not required to upgrade their equipment. If their current phone becomes unserviceable, their provider can only sell them an AGPS unit, but the customer can purchase a non-AGPS unit via private sale and have that unit swapped on their service. They cannot swap an AGPS unit for a non-AGPS unit.