Server Change

Hello Everyone, I'm using FreeRadius-2.2.9 in FreeBsd with InnoDB MySQL Database, mikrotik NAS's. Everything is working fine. Today I changed my Server. I stopped FreeRadius in both, then synchronized. In my NAS with users logged in, I just change the IP from my FreeRadius Server, and few moments later I receive an "Accounting-On packet" and all my users in recconect. How can I prevent it to happens in next time I need to change my Server IP? Thanks Aurelio