@netboy if you change the pool, then yes the devices that had lease with say IP .40 would get a new lease.. But its not going happen instantly.. The client would need to release and renew.. Unless you kill off the old lease it would still be able to renew that .40 etc..
If for example this .40 IP you want to know get something in your new pool that starts at .50 - would be to make sure that device does a release of the lease, and then gets a new one. Or you make sure you remove the lease on pfsense, the client might still continue to use that .40 until the lease expires on the device and he does a new discover and gets an ip in the new .50 pool.. But he wouldn't be able to "renew" it because pfsense no longer has that lease to renew.
If your device that currently has .40 is one of the ones you want with a reservation of .40, it wouldn't really matter - just setup the reservation for the .40 mac and whatever name you want to call it.