You don't. you can't set up the router to send some users to "this" ACS server and some users to "that" one. You'll have to set up the new ACS server to authenticate both sets of users, then you can change the IP address on it to match the old server and then take your old server offline. This way you don't have to change all your NAS configurations. Of course, if you only have the one NAS, then you cna simply set up the new ACS server to authenticate both sets of users, then change your NAS config to just point to the new ACS server.
If you do end up changing the IP address of the ACS server after you've installed ACS, make sure you change the registry value here:
HKEY_LOCAL_MACHINE/SOFTWARE/Cisco/CiscoAAAvx.x/hosts/hostname/ip
to be the new IP address. Note that x.x in the registry key above is the version of ACS you're running.
