I did this on my GX at 137K this past winter. Here are my recommendations:
1. Do drop the pan and replace the "filter" (really more of a strainer). The reason high-mileage transmission fluid changes can cause issues is that they can disturb sediment buildup at the bottom of the pan and circulate it through the rest of the transmission, so you have to remove the sediment before adding fluid and disturbing things. My GX had a nice film of sediment on the bottom of the pan, which I thorughouly removed with brake cleaner and a rag. The filter/pan drop is actually pretty easy (maybe <1 hr total with the remove and replace). After dropping the pan, put the same amount of clean fluid in the pan as you removed (more on this later).
2. Do the complete fluid exchange from the front of the vehicle (at the transmission color), all at once, rather than the partial flush. To do this you'll need to:
a) disconnect the in hose from the cooler, attach it to some clear plastic hose, and route it to a bucket/bottle that holds at least 2 quarts with markings. Put plastic over your fenders/paint so you don't get fluid on them.
b) have a helper start the engine, put a foot on the brake, and shift thru P-R-N-D-N-R-P until 2 quarts is pumped out into your bucket/bottle.
c) remove the out/return rubber hose from the cooler, attach some clear plastic hose to the rubber hose, and route it to an elevated funnel.
d) pour 2 quarts into the funnel; this will then flow back to the pan through the cooler return line. Note this may take a long time as the fluid is viscous - I'm talking like 10 minutes or so for 2 quarts. I saw a YouTube video where the guy made this a lot quicker with compressed air and a fluid jug.
e) repeat the process until you have 100% clean fluid coming out of the in line from the cooler, but not less than 15 quarts total added (including what you already put in the pan). My fluid noticeably cleaned up by 13 quarts and was totally clean at quart 15.
3. Once the fluid is exchanged check the transmission level using the plug on the bottom of the pan once the transmission is at operating temp ranges prescribed by the FSM, with the GX on level ground (check the frame rails with a level to verify, adjust as needed) and the engine running. I did this using a WiFi OBD2 reader and Torque, which pulls the actual transmission temperature from the ECU/PCU (WiFi reader is $30, Torque is $5, note you need to program in the equation to get the Toyota trans temp, which is easy and the equitation is listed on multiple Toyota forums). There is also a FSM procedure to jump pins on the OBD2 port; I tried this and could not get it to work. If the fluid level is too high, it will pour out of the fill plug, simply put the plug back in after it stops pouring. If it is too low, add new fluid through the fill plug at the rear right side of the transmission until it pours out, again with the engine running in park.
This is a messy, slow job, but it is not hard or technical. I would set aside around 4 hours, but less if you can figure out how to add the fluid back in using compressed air. If you can change spark plugs you can tackle this job. I strongly recommend the flush method above as you get 100% new fluid in the transmission and you get all of the sediment out of the bottom of the pan, rather than mixing new fluid with dirty fluid and disturbing sediment on the pan bottom. FWIW, I used Valvoline MaxLife HM fluid from Wal-Mart (claims to be compatible with Toyota WS, and is much cheaper). My old fluid was black at 137k. The GX shifts great (and it shifted great before).
