[sperry]

I would guess that a tranny additive won't overfill the box too much, usually it's only a few ounces of additive. However, Dean's right about the foaming... same thing can happen if you overfill your motor oil, and then you don't have nearly as much protection.

You might be able to open the plug a little so it starts to leak out just a bit and wait until enough slowly drips out, but as soon as you take the plug all the way out, you're going to have to drain the whole thing, since it's fairly impossible to install a plug back in there while the fluid's draining.