I sent my original PCM. I was able to park my 96 Jimmy for a week. I sent the PCM out on Wed. It arrived a week later on Thursday. Any 96 and later GM vehicle is easily upgraded.
Good reading on the upgrade.
You don't have to send the original PCM. However the cost is higher. They can send you a PCM for $159.00. After you install it, you will need a crank relearn procedure performed. The new PCM needs to know the position of the crank. Some people get by without the procedure. However, it is recommended so the PCM can work the timing properly. The procedure can be done by a dealer or a good shop. Costs an additional $70+. If you send the original PCM, costs are only $75 + 9.00 for shipping. The crank relearn procedure is not necessary because it is saved prior to upgrades. Once you upgrade any PCM, you can send it back for more changes or upgrades for free. You just pay for shipping.
Modifications that can be done: torque management is reduced, better tranny line pressure, torque converter lockup above 45 mph, egr valve cycle after the engine is turned off, code clearing, fuel map updated for octane used, speedo can be changed to match tire size, top speed limiter removed, and other small changes.
Read the link above for great details on the service. If I had a V-8 from 96+, I would do the same service ASAP.
My Jimmy has been updated with a new FI spider. I removed the poppet FI spider. I also removed the plenum air restrictor under the throttle plate. Tire size 255/60R17. Added a K&N filter. Everything else is stock.
The PCM is located on the passengerside fender well. Right next to the battery, bolted to the top of the overflow tank. Five electrical connections (all color coded) and four bolts. The PCM is easily removed.