Programming keys

When I got may 2003 earlier this month, it came with two original keys. One had no electronic guts in it at all, and the buttons were worn through. The second was complete, the buttons worn through, but the #2 and #3 button open their respective lids.
I see where everything about these keys is replaceable, and had the following question:
If the transponder is the device that communicates with the car's computer (immobilizer?), can I transfer just the transponder to a new key remote, with its new electronics (new circuit board), and not have to have it programmed? In other words, does the reprogramming reprogram the transponder, the electronics, or both? Has anyone done this?