Nah, that's not true!
You can, certainly with Nanocom, because I've done it (on a TD5 anyway)
The circuit board in the key has a shorter barcode on it, which Nanocom will take to allow you to program the key up.
With regard to your comment about wiping the old data, the key doesn't really have much other than its rolling code, and it's pin. What you actually do is program the car to accept that key, rather than program the key to unlock the car if that makes sense
David