L39 Guy:
The WAAS upgrade is a matter of upgrading the chip in the receiver (to do a 5 Hz update rate rather than a 1 Hz), the antenna (using the same cabling) and the software.
That's
assuming WAAS upgrade is the only thing that's required. Which is the wrong assumption in most cases.
In reality, a large number of A320s (and even A330s/A340s) do not have even vanilla GPS installed as part of the navigation system! No GPS units. No MMR. These A320s depend solely on radio updating (VOR/DME) or manual IRU alignment before takeoff.
(I believe the Air Canada aircraft involved in this accident was not equipped with GPS.)
Also, some of these aircraft have dual FMS which
are not GPS compatible. Total upgrade price == very costly.