Draining Transmission Fluid?

[RNEMESiS42]

Well, I don't know if the previous owner ever changed it, but I assumed he did since it was a nicely maintained car. I can tell he did, or tried, because I went down there and the bolt was all rounded off...so I didn't even try to get it off.
The reason I wasnted to change the tranny fluid in the first place was because whenever my car hasn't been driven for a while before going onto the highway, you have to push hard on the gas to make it shift into the next gear. The whole car "whumps." It didn't used to do this, until it sat for a year in a garage. So...I shouldn't change it? Just adding new fluid is good enough? If I'm still going to change it, I wanted to replace the drain plug, too...but I don't know where to get one that fits. This is the closest thing I could find, but it doesn't mention what size it is at all.