I would imagine that a better project (and much easier, if you're just starting out DIY-ing with headphones) would be to replace the broken plug (not jack, the jack is what the plug goes into) on your XB700's with a new one. Buy a simple 3.5mm plug, find a tutorial, and replace it. If you haven't done much with headphones before, replacing a plug seems like a much less daunting task than building a pair of headphones.
Replacing a plug is a much less challenging task than dismantling a headphone, and much less can go wrong than when you have an entire headphone's worth of parts strewn about your desk. Thin little internal wires can snap easily, and all sorts of bad things. With plug replacement, you worry about only one wire and one plug.
Also, no two pairs of headphones disassemble the same. Especially in the case of the XB700, which I believe has its ear cushions glued onto the plastic frame, so it's a pain to get off, and you have to glue it back on carefully. Not a good choice for a first time. But plug replacements are basically the same across headphones.
Another thing too, the XB700 has a unique sound signature, which I expect to be considerably different than the ATH-M50's you are planning to get. The XB700 is designed for pure bass fun, while the M50 is more well-rounded, though still a bit bass-heavy. It's not a crime to have more than one pair of headphones, I have three, and am hopefully getting a fourth soon.
TL;DR:
Just replace the broken plug on your broken XB700. You'll have a MUCH higher chance of success for a first-time project, and two working pairs of headphones.
