Man I knew you were going to say they were glued. That sucks. If you look at my link below, you could pull the corner pipe right out on my tank. I forgot what the called them, but after we drilled it, we used threaded a plastic hex bolt and washer with a rubber seal on tank. You could just stick the pipe in the hex and you were fine.
ANYWAY, without really seeing it, I am tempted to say you could cut another pipe to the shape of the underflow hole and silicone it in the hole. It may not be pretty, but it might work. Or maybe you could take a hacksaw blade and cut off the outer baffle pipe? Hard to say going blind, but maybe it gives you some ideas.
Good luck!