Need help with changing the ATF.

I own a 1992 Galant, with a 1996 4 speed auto Hyundai Sonata gear box in place of the original Galant box. It fitted nicely and now all it does is move in Reverse and no other direction, no matter which forward gear is selected. This box came with no oil in it, because the freight people didn't want oil everywhere. So that's fair.
This is where I need the help
What I went and did was put 'Valvoline Dexron III' in the F4A21 automatic gear box. I've been told that only "Castrol TQM-ST ATF", should be put into these gear boxes. Now I have a gear box with the wrong oil in it.

So I'm about to remove the wrong oil. How do I do it after removing the gear box plug and draining the oil from there. There must be other areas where this oil is 'trapped', or will this be all that's needed?

Please can someone help me and send me in the right direction.:frown:

Regards,
Mark A
Swansea Tasmania
Aust.