I've done that everytime I've replaced a PVR (501s, 508s, 942s) because there were always unwatched programs on the HHD. As long as you disconnect the satellite feed (s) BEFORE you call in to deactivate, the receiver can't get the shut down code.
It will just sit there trying to "aquire" a satellite signal but you can just press the PVR button (or menu and go through the choices) to get to the list.
I have heard there are some receivers that will eventually shut down all use after a certain amount of time of not receiving a satellite feed but I've never run into that problem as I've always either watched or copied everything from the HDD within a month or two. I've had a 508 hooked up that way for about couple weeks now in the family room and I'm betting it will take the kids a while to watch everything they have recorded on it, so we'll see.