For some reason there are 2 drain plugs on the transmission sump handily referred to as No1 and No2 by Lexus, no idea as to why when one seems to suffice on my LS400.
Conventional method is to remove the drain plug(s) measure what comes out and then refill the same amount using fresh fluid then repeat 5-6 times to replace the majority of the fluid over a short period of time.
The problem may be refilling as I cannot see a conventional dip stick on this transmission, I am aware that some manufacturers require diagnostic equipment to set the levels and specialised tools to do the job.
Perhaps a case of going to a dealer for the job but perhaps not Stockport.