Silverado Transmission Service. How to Change/Flush the Fluid/Oil by Dropping the Pan.

My Silverado was due for a 100,000 mile transmission service so I needed to drop the pan and change the filter. I was interested to see what the bottom of the pan would have in it (shavings, etc). I bought a new filter from Autozone and some transmission fluid from Walmart.
This was the first time I ever messed with the transmission and I was a little leery. I had read that the bolts on the transmission pan might be super tight and difficult to get off. I sprayed each bolt with some perpetrating oil the night before. The bolts came off real easily. The only problem was that the gear selector/shift cable was right in the way. The pan would not com off because the bracket holding the cable was in the way. I bent it out of the way (not smart) and finagled the pan around the tail pipe. The pan was really pretty clean except for some metallic looking gunk stuck to the magnet. The filter and gasket went on easily and I put the pan back on and bolted it up. Way too easy...so I thought. I put in 4 quarts on transmission oil and started the truck up so I could use the dipstick to see how much more oil I needed (it takes 5 quarts to refill the pan). I shifted the gears (P R N D 1 2 3) then tried to shift it back up....the shift handle moved up without anything happening. WTF! I looked under the truck and saw that the gear shift cable was wobbling and the clip that held the cable to the bracket was on the ground. A plastic piece had broke off the cable and now I was screwed. I needed a whole new cable ($160 through a GM dealer). The cable was only available on-line (too long to wait) or at the GM Dealer (expensive). Autozone, carQuest, O'Reilly's/Murray's, NAPPA, Discount Auto Parts, Pep Boys, etc..nobody carries this part. I bought it from the dealer so my cheap transmission service turned into an extra $160. Lesson learned, don't mess with something I don't know anything about. I have done a lot of repair/maintenance work on my 2005 Silverado in the past year, but I fully intend to keep it to the basics. I can do my own brake service (new pads, rotors, calipers) and have fixed an electrical problem with the A/C-Heater fan but I will now on keep it to the routine maintenance schedule and pay the local repair shop to do the tough stuff.