2002 toyota sienna change transmission fluid and

The drain plug is on the bottom of the transmission. Its torqued to 49 nM.
1. Remove tranny oil pan drain plug
2. Drain old fluid (roughly 2-4 quarts)
3. Reinstall drain plug.
4. Measure amount drained in a 99 cent orange juice pitcher from Walmart
5. Pour back the same amount drained using Toyota Type T-IV fluid (available in 1 quart bottles from a Toyota dealer.
6. Start engine and shift slowly through all the gears ending in Park.
7. Check fluid level on the dipstick. Add fluid, if necessary, to bring the level to a point inbetween the COLD and HOT marks on the dipstick.
8. Go for a 20 minute drive and recheck fluid level. Add fluid, if necessary, to bring the level up to,
but not beyond, the upper half of the HOT range on the dipstick.

As in 1973, NO FLUSHING is needed and NO FILTER REPLACEMENT is needed, NO PAN REMOVAL is needed. As, as in 1973 todays Toyotas have a metal mesh filter screen that doesn't ever get clogged except in rare cases of transmission operational abuse, including failure to ever change the fluid for 150,000 miles or more.

Hi,
I had transmission fluid drain and fill last year and had not driven more than 6K since the transmission fluid replacement andfound out that the fluid color getting dark.

My question is now how I should change it now, should it be completely flush or regular drain. I have heard that complete flush makes the transmission seals weak, that is why not recommended, is that true

please help