LED projectors work with three seperate LEDs for each primary color. This allows a huge color gamut (way larger than the one of Sonys Xenon for example) and in conjunction with a good CMS, you can get accurate colors according to older videostandards too.
Also, the red defecit of UHP-Lamps is non existant with an LED-projector, so no brightness and contrast decrease because of calibration occurs.
All above is already proven by the actual Vivitek LED projector. We expect it to get even better with future models once the new technology catches on.
But it is unlikely that it will be possible to upgrade older projectors. It will not work with the dichroic mirrors, cooling paths, light emitting size, etc. etc. Also, you would have to modify the whole power unit of the projector, as these LEDs need low voltage (4V) but up to 30 Ampere! In addition, they are pulsed... and so on and so on...
Sorry, you have to wait and buy a new projector when you want LED