They have done that in Canada in preparation for the Olympics, you know, all those Europeans coming in with GMS phones, they nearly had to. Basically, they still have the CDMA base, with an HSPA layer on top of it. The different technologies are all about the shared use of a radio frequency channel, and the most efficient way to do it. It's not all that different from NAT on a computer network, actually.
4G is a GSM standard, but can be overlaid relatively easily onto CDMA networks. At least from the phone end. Lots of stuff has to be done on the carrier end, but not a bunch of new towers or anything like that. Data gets translated into whichever "language" is needed by the overlay.
The phone companies in South Korea and a few other places moved to a hybrid CDMA/HSPA network a couple of years ago, and it's what Verizon will be doing, as well. It makes the path to LTE much easier and relatively seamless. CDMA is definitely on its way out, at least in its current flavor, since what it will evolve into will be the 4G thing where CDMA voice will be ported over to 4G. At that point most all phones will work on all networks. What remains to be seen is how willing CDMA carriers are to allowing SIM cards. Once that happens, phone exclusivity goes out the window.
The thing is, except for Qualcomm's exorbitant licensing fees, CDMA is superior in many ways, technologically speaking to GSM, and the 4G standards will likely be a hybrid of the GSM and CDMA technologies. HSPA already is to a degree (there are about 5 different flavors of HSPA, each with additional letters). The biggest differences between GSM and CDMA is international compatibility (GSM wins, though that is somewhat of a misnomer), how the phones are activated (CDMA-Remotely by the carrier, GSM-locally by inserting a SIM card, GSM wins), call quality and reliability (CDMA wins), and broadband speed and reliability. AT&T's 3G broadband service, for example, is theoretically quite a bit faster (twice as fast) than the 2mbit broadband of EVDO's 3G, but that's .only under circumstances, and it's something that happens very, very rarely with current networks. In the real world, a headless Luke Wilson notwithstanding, both networks run at about the same speed, which tends to be somewhere between 500kbit and 1.5 mbit. In really good areas I'll get 2mbit, but mostly my downloads are in the 1.2 to 1.5 mbit range.
As for the international compatibility being somewhat of a misnomer, that's because many people think North America is the only place you'll find CDMA, and it's GSM everywhere else. It's Western Europe where GSM dominates, but in Eastern Europe, Russia, Norway, Sweden, Denmark, a few others, it's CDMA (2G, 3G, or both). Most of South and Central America is CDMA, as is most of Asia and the Pacific (China, Mongolia, India, Pakistan, Indonesia, even New Zealand). Africa has CDMA all over the place. Many of these places have GSM, as well, but some are strictly CDMA. So it's not like you can't take your CDMA phone overseas, you can. You just need to stick to places like South Africa, or Yemen, or Tajikistan (gesundheit).
