Interesting thought. I actually owned a pair of HD429s before I got the Sennheiser Momentums (I got the momentums for free as a part of Sennheiser's original momentum promotion contest thing that they did for a few months after my HD429s broke). I personally think however it'd probably be a better idea in the long term to just suck up and stick with the Momentums and just mod them for better comfort.Sound quality aside, I can say that the HD439's would be more comfortable, which is why I recommended them, and because their sound quality is generally praised as being far beyond it's cost. You could sell the Momentum's, pocket the difference, and put them toward a newer heapdhone down the road. Would SQ take a step back? Probably. That's what I meant about a mulligan, though I understand not wanting to take step backs in SQ.
What I am most likely to do is hold onto the Momentums and use them as long as possible and then when the time comes I'll sell them and put that cash toward whatever new pair of headphones i want to get (eg: HD650 or something in that price range).
Thanks so much for the suggestion though. I'm sure others looking at this thread would definitely find it interesting and worth considering.