At what temps does the fluid bypass the cooler?

I am getting ready to change the transmission fluid. I plan to use Mark K's procedure. I want to change the internal filter and inspection the magnetic and clean the pan etc. since the truck is new to me. But I was wondering if I follow Mark's procedure and disconnect the return line, run the truck to pump out the fluid etc and stop after the first cycle to drop the pan, change the filter and clean the pan/magnetic...will the fluid still in the transmission cool so much it will bypass the cooler and return line I have disconnected? At what temps does the fluid stops bypassing the cooler?

Griz