To be clear, are you talking about activating the purchased receiver on the same account you had when you recorded the shows to the EHD's, and then disconnecting the receiver from the satellite signal to make it portable? If so, then that should work, at least for awhile before it loses the authorization. It is a good idea to reconnect the receiver to the satellite signal on a regular basis to make sure it keeps the authorization for the DVR functionality.
Which type of receiver was used for recording the shows? Some people have had trouble trying to use a Wally to view shows that were recorded with a ViP211, so it is best to stick with the same type of receiver: ViP211 series to view recordings made by ViP211 receivers, and Wally to view recordings made with Wally. If the recordings were made by a DVR with an internal hard drive, and then transferred to EHD, then neither the ViP211 or Wally will work. You would need a DVR receiver such as the Hopper Duo (or any other Hopper or ViP DVR) to watch them. Alternatively, you could transfer the recordings that you want to watch from the EHD to the internal hard drive of the DVR, and just take the DVR with you.