[Zeke]

"...and the economic position of both parties."

There's the key phrase right there. But, in my non-law school and humble opinion, that would not necessarily put your ex's new husband on the hook. You would likely have to prove your ex has a bunch of money of her own to modify the shared split. Now that's another interesting word: shared. Does it say 50/50 all the way down the line, or is that 'modifiable'? If she is indeed well off in her own right, it should not be your legal responsibility to step up to a much higher cost of education simply because your ex can afford a higher "share" dollar wise. Morally, this is a sticky wicket going every which way.

That's the tack that I would take. (not tactic or tact, but you could say tactic just not tact)