ArkansasGS300

I have a question, I purchased a non running GS300; I had to tow dolly it 150 miles, according to the manual im able to do that up to 200 miles...

When I got home I started working why it wasn't starting, turned out to be a bad ECU, purchased another ECU from a 2000 GS300, thats another question...on to my main question.

I checked the transmission fluid, it was like way up the dip stick, wiped it off, stuck it in again, way up on the dip stick. Crawled under it and noticed one of the lines was taken off the transmission, i believe it goes to the radiator...My question is, if we towed it with the rear wheels on the ground in neutral, does this work the transmission and in result work the pump. Did this remove all transmission fluid from the system and dumped it in to the transmission pan? Is that why it was so full? I'm going to drain it, but i know when you drain it you only drain whats in the pan. So should I drain it hook up the other line, and fill it up with like 3-4 qts and run it and check it again, or do you think this possibly messed up the transmission. I'm just looking at what is the best way to make sure there is the correct amount of fluid in the transmission.

Note: the fluid was dark and needs to be changed, I know from my GX470 that you don't want to flush the transmission, as this can cause more problems.

Also if the ECU now reads 2000 GS300, will this cause a problem? Is there a way with Techstream to be able to fix it? Im in the process of putting it back together. But want to preserve the trans if its not bad, or cause more damage to it.