Should you change all of your transmission fluid?

That sounds pretty strange to me. I'll admit I don't know a lot about automatic transmissions, but with a manual gearbox I would always drain all the fluid when changing it. As gearboxes wear, small metal filings accumulate, and one of the reasons for changing the oil is to get rid of these filings which, if left to build up unchecked, cause even more wear. Most gearboxes I've dealt with have also had a drain plug on the bottom (so you can't only drain half if it anyway), with a magnetic element to collect the aforementioned filings

Sounds like "Cleatus" the slack-jawed yokel changed your fluid.

As stated above, there are lots of tiny little metal filings floating around in that stuff. The more that stuff moves around the more it scratches and wears out your transmission. Then you have half of your new transmission fluid filter life because, instead of all new CLEAN fluid, half of it is already full of filings that will now get trapped in the filter.

Do not go to that mechanic again!

I have a Toyota Land Cruiser with an automatic transmission. If I drain the transmission with the drain plug, I only drain 40% of my transmission fluid. The rest of the fluid is, in my case, circulating through the transmission fluid cooler, transfer case and gearbox. I only drain what is in the pan. The Toyota Factory Manual does not state any full fluid flush for my vehicle. (Here is a "Rodney Flush" that does change all the fluid). Here's a DIY for the RX300 that flushes all the transmission fluid.

Bottom line: if the Lexus technician left 50% of the oil intentionally, I'll be worried. But, I bet he has done a "drain & refill" that left some old oil in the system, and I'm sure that's what the Lexus Factory Manual has stated to do.

If unsure, call the dealer and ask for more information.

The old lore goes that if you change it all out, the shock of all the new fluid and detergents will knock crud loose and cause the transmission to fail. I've heard it many times but never actually seen any evidence that it's true.

Given that no Haynes or factory service manual mentions anything about it, I shelve it up as false.

That is completely wrong. The automatic transmission fluid is designed with a special viscosity to ease the changing of gears. This fluid starts to lose its standard properties and should be changed to keep the transmission parts and its performance at its peak. Mixing old and news fluids wont give you the ideal viscosity and the reduces the performance of the transmission system.