My advice is to start with the back bolts of the transmission and then work your way around to the front bolts. I've changed transmission fluid at least 15 times on the 4t45e tranny's and this is by far the easiest and least messy when doing it with the front end lifted up.
I have never changed the filter seal. I would change it if it is damaged or hardened but I have never come across that situation so I only replace the filter. Be prepared if you have to though.
I believe the pan bolt torque is 106 in/lbs. Over tightening will damage the pain. Use a licensed Dexeon VI ATF, Walmart supertech is licensed but not always the cheapest. Name brand, store brand, Walmart brand, it all works the same. Look for the J-xxxx license number on the back of the bottle and shop however you'd like to!
Do not use a dex/merc, or dexron III fluid in tranny's originally spec'd for Dex VI, I've done it and experienced shift problems and had to go back to Dex VI to correct the problem. Stick with the recommended fluids!
Dexron VI/Mercon LV dual licensed fluids are showing up in the market and they work well. Walmart now has a Dex VI/Mercon LV licensed fluid and I've used it. I think Mercon LV has better anti shudder additives than dexron VI, either way the dual licensed fluids are a fine product.
I recommend changing the fluid and filter, drive a few hundred miles and then change the fluid again (leave the filter). This will result in a higher percentage of fresh fluid and the second drain results in the removal of suspended gunk in the fluid.
I currently have 196,000 miles on my G5 Auto and I change the fluid on it and all my other vehicles based on the "SEVERE OPERATION" guidelines in the owner's manual. For GM's that's usually at 50k miles. The advice I gave is from my own experience. Also my tranny is original and never been rebuilt so I feel confident in my ritual.