To add to what Shannon described above:
My understanding is that technicians have been instructed to install the Xfinity Home wireless router as the primary device connected to the cable modem.
I have the WAN port of my Xfinity Home wireless router (Netgear unit) plugged into a LAN port on my existing Asus RT-N56U. The system works fine for me in this configuration.
It should be noted, however, that changes to the configuration post-install could make it more difficult for Comcast personnel to help troubleshoot any issues. One reason they try to make the install very standardized is so that our Customer Care agents know what to expect when attempting to help you when you call for support.