to force a DHCP cycle, you have to 'bounce' the client's port. This normally gets done via a CoA PoD (Change of Authorization, Packet of Disconnect) when you're changing VLAN's via Profiling. In a normal L2 authentication it's not required because the client may receive the VLAN via the Radius Accept message, and then only will the DHCP cycle have any effect.
I haven't had much experience with Profiling, so perhaps someone else can give a real world example.