Which plug are you "plugging" (sorry for the pun) the old "the plug" or the "spark plug"? I ended up just getting the sparks because Circuit C. had them on the shelf and I had an gift card rotting in my wallet. So here's comparision:
Fit, nearly the same, the new ones are a tad bigger (deeper). (It should be noted I don't use the Koss supplied foamies, I make my own from the yellow foam ear plugs {hardware store}, trim them back a bit, put a hole through with a hot nail and you're set).
Sound, quite a bit different, much more bass which was already great in the old plugs now it's too much, had to set my EQ different, actually ended up listening with treble boost on for much of the time whereas before I had bass boost on most of the time, the Rock setting sounds good too.
overall, pretty similar, very good sound, cheap, I actually don't like that they're white, too noticeable, I'd prefer the old black like "the plug".
Maybe I'll still ship my old one's back to Koss for $6 I can have a backup pair.
Yep, these are great plugs for the average Joe.
I use them mostly on my motorcycle, open helmet, loud pipes and the sound I get is still awesome, even on the highway, that's how well the foam can block other noise. (homemade foam, never had luck with the Koss ones)