Many network routers masquerade as PCs by cloning their MAC address. See
MAC Addressing. Cloning on a PC might work for preserving MobiPocket PIDs, but I have never tried it. I don't think this is DRM circumvention if you do this because of a machine (or Ethernet card) upgrade. It obviously is DRM circumvention if you have several machines with the same PID (from cloned MAC addresses).
As others have pointed out, providing the e-book store you bought from is still open you can change the PIDs allowed on MobiPocket e-books and download them again for the new device. If the store closes, techniques like cloning (or outright DRM breaking) are your only alternatives to get the e-books on new devices.