Auto transmission fluid - all at once or partial regularly?

This last weekend I changed the fluid in the DDs ('11 Mazda3,'09 G37) and both were shockingly burnt.

The Mazda is pretty easy yo change, drain plug is easy to access and it is a top fill. Doing a few drain and fills is not to difficult. The G37 is a bottom fill plug and more of a pain to do as filling requires leveling the car, getting the fluid to a middle temp (104*) over filling and let drain out to proper level.

On the G, this is not a big deal to add a drain and fill to each, or every other oil change. But to do once, drop, drive, re-level, drain fill, rinse repeat 3-4 times is most of a day affair. I would do it at the dealer, but I am not entirely sure that they did it last time. I have never seen fluid so bad after 40k that it was in since I last had it done at the deal just over a year ago.

So the question is, is it inherently better to run the fluid the full life and swap completely or to constantly freshen up a % equaling or bettering the standard fluid replacement timeline? These cars live in Phoenix and the G regularly sees lots of stop and crawl on very hot roads.

Install coolers, drain/fill the Mazda, full exchange on the G37.

I usually only see Really Bad Fluid in vehicles where the cooling system is trained to run low. If the radiator is only half full with the engine running, the engine may still run cool enough to not overheat but the trans will run very hot. (This offer only applicable to side flow radiators)

Change it all now, do a regular service more frequently from here on out.

Add in a bottle of Lube Gard "red" (O'Reilly carries it) after you have all new fluid in both cars. And install external coolers in series with the raditor coolers. If you haven't done any real damage yet, the external coolers plus keepong the fluid fresh and adding Lube Gard can make an auto box last an incredibly long time.