I'd love to see your source that it would cost "millions of dollars" to throw a CDMA chip into a phone. Every other carrier has had no problem doing it, and they don't have near the cash flow Apple has. A few that come to mind include: Palm, RIM, Nokia, Motorola, HTC.
Besides, why would a CDMA carrier switch to GSM when GSM is a dying technology?
You're very misguided.
I dunno if you ever taken a part an electronic device, but they are hardl "thrown" together.
To make an iPhone with a different antennea would require a whole new fabrication plant to stamp put the logic boards, and a new assembly to have the new antennea work on the new logic board.
Apple notoriously keeps their costs at a minimum, opening new assembly lines to appeal to a few million possible users rather than say, putting the same time and effort into expanding the number of markets the gsm is sold in or advertising more effectively in china is not cost effective.
Additionally, CMDA is the archaic technology, hence it being slower and less well designed (audio and data on one connection). This is why AT&T can use mostly software upgrades to advance to LTE while verizon is stuck doing a major overhaul. Fortunately for verizon they put more money towards cell development than AT&T and so should be able to make the switch about as fast of not faster than AT&T. When LTE is a standard, then the iPhone comes to verizon.