I have set this up a few times before. have you lines route to a phantom extension in day mode and to AA in night mode. then use the remote programming feature to change call forwarding on the phantom. You can also program a button that will use the remote progamming feature, but it requres you to enter the on-line monitor.
Example:
I created phantom 2101 name it whatever you want
in Devices and features\co trunk groups set the day ring in to single and 2101 set the night ring in to your auto attendant
a hunt group for each of your day modes
on the phone you want to be able to control the modes, program two of the keys as station speed dials and on as a night button
enter on-line monitor mode from the view menu
under devices and features\phones select the phone you programmed the speed dial keys on
you can now edit the speed dials. you need to do it this way becaus it requires that you put a # in the speed dial string, when you are programming from the phone # saves your entry
program the speed dials as 359(phantom ext #)#355(hunt group #)
for example, mine look like this 3592101#3552000
you also need to remove the password from the phantom extension in devices and features\phones\<phone>\passcode
and you need to program the speed dial label from the phone.
once this is done you end up with 3 buttons on the phone. the night button will toggle between day and night and if it is day mode the calls will ring to whatever speed dial you pressed last, unfortunatly, there is no indication as to what "day" mode it is in.