If the current controller is a local do the following to make it a master :
(controller) (config) #no masterip
And then it is going to ask you to reboot the controller (save the config)
After that add the new 2400 as a local on the current 2400 (master) do the following :
(controller) (config) #localip 10.10.10.2 ipsec "KEY"
And finally on the new 2400 (local) add the current 2400 (master)
(controller) (config) #masterip 10.10.10.1 ipsec "KEY"
Both have to be the same key