As I posted earlier, I didn't realize the 5L50 and 6L50 were completely different designs. As a result, the pan is totally different and there is also no drain plug. The procedure is generally the same. The filter looks very similar.
There is a plug on the pan but it is actually a fill plug. I opened it and some fluid came out. The rest of taking down the pan is the same as any other car and made somewhat of a mess.
I cleaned the magnet, pan and gasket. I reused the gasket as it looked fine. I'll keep the new one I bought as a spare. It is not the same as the 5L50. It does use the double O-ring style seal but the construction is different. The 6L50 pan also has like 18 bolts and a fluid and filter change is 6.5 quarts, which is 1.5 quarts more than the 5L50.
Once everything is clean and the new filter installed, I installed the pan with the old gasket and tightened the bolts. Then I looked at the fill plug on the pan and realized there would be no way I could stick my pump hose nozzle on there and fill anything. It is vertical and the fluid would come right back out of the hole. I remembered the FSM showed a fill plug on the case, kind of like where a dipstick would have gone. But it was very hard to reach, behind the exhaust and front driveshaft (of course). I put my fingers up there and realized it was not a bolt, but some kind of rubber plug.
Using a pry bar I was able to get the plug out. It works kind of like those plastic push rivets. It has a central plunger which expands the plug after installing it, to keep it tight. Once I removed that I was able to stuff my pump hose nozzle up there.
Fill procedure is the same as the STS. Get a bunch in there, start it, run through the gears, then wait for the trans to get to the proper temp range (88-122 or something). It actually has a sticker right on the pan that says that, and also says Dexron-VI. It took about 6.5 quarts by the time fluid started to come out that pan plug, so I reinstalled it. Then I had to shut the engine off as the exhaust was too hot to stick my arm back up to put the fill plug in. After it cooled a little I was able to do that.
Oh, important note: the fluid looked really good and smelled normal, unlike the STS fluid which was very dark (and which was not a Dexron-VI factory fill). I'm not sure it shifts any differently but it is good that I was able to change out like 55% of the fluid and filter. 120k.