Back when I still had an automatic trans, I looked into getting the computer reprogrammed and it's not possible.
However if you really want to change it, I finally ended up getting a "pulse control module intended to quickly send a 12 volt pulse when a couple of leads are energized. Typically these are used for power locks in combination with an alarm/remote. I energized the box with power from the accessory leads on the ignition switch. This in turn was spliced in so as to send the pulse to the ECM (just like hitting the button). Worked pretty good, but considering it took me an afternoon of futzing around it was probably easier to just get in the habit of hitting the button