Hi
this is possible, but please state what version of xbox you are using. and was the 2.6 chip installed using a pin header install?
if so, i have done this myself and it did work..
get executer 3 chip, plug it in to your pin header port where your old 2.6 chip was, fix up the included bank switch and panel switch and bingo you should see a flash cromwell screen. X3 installed.
flash the executer 3 chip with the X3 1959 - 1024k bios setting your bank switch to on on off off off and bobs ya uncle.
NOTE: if you dont get the blue cromwell flash bios screen when you have installed the x3 chip, please dont try and and re-soilder at that point check the chip is seated properly before you do anything else.
anyway hope it helps.