There are separate drains for the pan and torque converter. Turn the engine in the direction it normally turns, a ratchet with 27mm socket on the crank pulley bolt works well, and look for the drain plug to be at the lowest point in the torque converter's rotation. IIRC in a 126 the guide rod mount crossmember has to come off or there's poor access and fluid will splash all over the place. FSM says to use new bolts but most folks reuse the bolts with blue Loktite after cleaning all the threads.

There's a method to filling the transmission if you drained the torque converter. Lots of discussion in the archives. How I do it: pour in 4 quarts or so, start the engine, add fluid in half quart increments (pour, pause, slowly run through the gears), until the transmission engages. For me this is a critical juncture because then I can back off the ramps. With the car level, continue in cup increments with the same pausing and shifting until the fluid is about 1cm below the lower mark on the dipstick. Drive 20 miles, make sure fluid on dipstick is too hot to touch, check and correct fluid level. YMMV.

James, on the '97 C2500 'burb 6.5 we got with 150K miles, I replaced the filter then flushed a couple of gallons through the cooler lines. Still okay 25K miles later. Bill Heath's advice would be more relevant if the fluid was in there waaay too long. It shouldn't be a problem if the transmission has seen proper maintenance. Flushing the transmission is another matter. I wouldn't recommend that particularly on a mistreated transmission.

Sixto
87 300D^2