The LAST thing you EVER want to do is drain/refill, which will introduce the maximum amount of the enemy (oxygen, heat being the other one) into the otherwise relatively-sealed system.

How's the fluid? As long as it's not black/tar or nose-curling burnt-smelling, I wouldn't worry about the transmission either way. (Redness of the fluid means nothing to the transmission or its longevity, though it gives worry-wart owners s a semblance of peace-of-mind.)

Since the AT pumps its own ATF all the time anyway, my preferred method is to unhook the cooler supply/return lines and let the AT suck in fresh/new Dexron and spit out the old all by itself until it comes out clean.

THEN, RESET the AT's ECU, to clear its memory/history (of how it'd learned to shift smoothly using the old, probably grabbier, ATF), so it starts out in learning mode just as it did when it was 'born'. That should mimimize retraining shocks/jolts (or, worse, slippage!).

If you did not reset the ecu (I've long forgetten which fuse# to pull or for how long), and the new ATF behaved sufficiently differently from the old, you'd be tricking a 'smart' system, which could be asking for trouble if the system did not learn/compensate well enough soon enough.

Roll on,

- Dave; '95 854T, 143K mi