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. All that is being done is the GPS sensor in a multi-sensor MMR is being upgraded to incorporate the WAAS correction which, by the way, is on the same frequency as the USAF/GPS satellites; Galileo, incidentally, uses the frequency and is interoperable with GPS/Glonass/WAAS.
Doing upgrades for other items such as CPDLC, etc is beside the point. It is important to note that in order to be ADS-B compliant after 2020 in the US, a WAAS position will be required so, unless the air carriers snivel for an exemption, they will be upgrading to WAAS anyways.
With WAAS grade receivers, precision approaches to virtually every runway in North America becomes possible and with Galileo every runway in the world.
Compare this alternative with upgrading MMR's for Local Area Augmenation System (LAAS) which will require VHF receivers, software, certification, etc and at least $3 million per airport for the LAAS box to get the same results as WAAS/Galileo for airline only type aircraft and the choice is clear.