As far as avoiding a FLUSH of the transmission~~~~~~~~
I have read on this and other forums that a trans. flush is potentially more harmful than helpful. They seem to be of the consensus that simply dropping the pan, draining the fluid out, replacing the filter is the best way to maintain the trans.
1] It is likely possible that the flush [as mentioned by another respondent above] merely recycles and treats the existing fluid. That is akin to not doing anything in the first place, lol.
2] The flush requires unhooking the trans. lines to the trans. cooler [or radiator, car dependent]. Possibility that the hooking back up neglects to replace any seals or O-rings, or causes damage of some sort. Essentially, leaking could occur.
3] Since flushing doesn't usually provide removal of the trans. pan, you are still left with the original [not a new] filter. Somewhat akin to changing the engine oil, but leaving the old oil filter on. Also, any sediment or "sludge" that has collected at the bottom of the pan is still there.
4] And since the flush uses pressure to flow fluid into and out of the trans., the pressurized fluid can and usually does stir up that gunk in the pan and also various recesses and nooks/crannies in the trans. body itself. Thus, the new fluid can become contaminated, or the filter can become clogged or lose flow ability.
With that said, dropping the pan, while not always something a DIY'er can do, reduces the risk of problems dramatically. The fluid gets changed, the filter gets changed, hopefully the pan gets cleaned out.