[slomotion]

First of all, before I started pounding on the leaker, I'd do a "controlled Leak" and drain the coolant into a suitable bucket. at least you could keep from flooding the floor. You may have others that are borderline, but I wouldn't worry about them until another one started leaking...you're not likely to get a catastrophic failure and loose all the water all at once.... it'll most likely start as a puddle on the floor where you'd least expect it. Having said that, if you do have another one leaking, I would certainly change them all out at that point. Pulling the engine sounds extreme for such a simple job, but it's the only way you're going to be able to access some of them. Now, there's two schools of thought on the proper way to install a "freeze" plug. (They aren't actually designed to pop out if your coolant freezes and protect the block, although apparently this has happened, they're plugs to fill the holes that allow the block to be cast which is a whole other story.) One school of thought is to install them concave side out with some sealer on the outside diameter surface. The other way is to put them in flange-first and seat them against a ridge that you'll find inside the hole, then just bump the center of the "plug" with a ball-peen hammer to expand the outer edge against the land inside the block.
I've seen them both ways...one way there's a depression, the other way the block appears smoother. I'm not sure which way is the "right way", but someone on the board will tell us...........

Good luck.