DSL is very different then FIOS. With DSL, either you have a normal DSL modem, which doesn't have any router functions, or its a modem/router gateway, but it can be put into bridge or "transparent" mode. This negates those router functions and allows the 4500 to be the one and only controller for those functions. With FIOS, every modem is a modem/router gateway. But unlike DSL, that gateway performs some important duties for your digital cable system. If you put the gateway into bridge mode, it can't perform those duties, and some things like your guide for your cable stop working. If I am wrong about this, please someone correct me, but thats it in a nutshell? Will it work anyway? Probably? Could it be problematic? Likly. Not saying you shouldnt switch to FIOS, just saying you may want to do your homework on Verizon's forums first and mention it to the installer.