Transmission oil change

[JMBT]

I am getting ready to drain and fill my trans and rears too and I have decided to switch to the thicker SAE oils in all the boxes. My question....when switching from synthetic 50wgt. To SAE 80-90 do I need to do anything special to flush the trans first or just regular draining gets enough of the synthetic out? Same on the rears, just drain pumpkin the normal or something special to get all the synthetic oil out?

I am guessing the system would have some tolerance for the synthetic residues that will remain on gears, bearings, and in little nooks and crannies?? Right??