NM100 and the NC300 CD add on. The new unit by itself will not fill the hole but there is a frame kit made for exactly this purpose. When using the NM100 and NC300 CD add on you will actually have to make the hole slightly larger.
If you replace the room stations (you may have to depending on what you have) with the newer model you will have to use the frame kit they make for them as well. I would suggest this because the newer model stations have a very nice sound when distributing music through them. Well worth the money.

The NM100 system is a full-function intercom and music distribution system designed for retrofit applications in residential use. The ‘Master’ unit can replace older intercom systems that use three-wire or four-wire connections to the room stations. The system supports up to 15 remote stations when retrofitting a 4-wire system, and 13 remote stations when retrofitting a 3-wire system.

[Link: nutone.com]