I have a Sprint Optimus S with which I was attempting to change the ESN to that of my Touch Pro 2 (on it's last legs). I have been successful with changing pretty much anything in the device except for the ESN, using CDMA workshop. For some reason, it allowed me to zero out the ESN but now it won't let me change it again. Ideally, I'd like to put my Touch Pro ESN in to it but if I could at least change it back to the factory issued one, I could fight with Sprint for a few hours to get them to do an ESN swap.
At this point, I have a decent phone, in great shape, with an all zeros ESN.. I couldn't find a thread that touches on my particular situation, even on other forums..