Yes, it is possible UE can switch from unicast bearer service to MBMS bearer service when UE enters into the MBSFN area which is providing the same MBMS service it was getting through the dedicated unicast bearer. UE exchanges the messages with AS (application server) though GC1 interface. After getting the indication from the UE, application server (AS) stops sending the MBMS data through UE associated unicast bearer and it asks PCRF to delete the unicast bearer.
Before sending the indication to the AS, UE starts receiving data from the MTCH hence there is no interrupt in the receiving MBMS service.