Another vote here for TearsofLove's idea. If the lawyer thinks you can get somewhere with this, AND it's worth it to you, go ahead and pursue it.
But how much is it going to cost you to pursue this? Even if you can afford it, wouldn't that money be put to better use, say, buying better presents for the kids for Christmas? Or tucking it away in their college funds?
Think about it - if you do the big Christmas morning thing on Christmas Eve morning, the kids get two chances to have a fun Christmas, and you don't get the "leftovers". It may or may not make the kids less excited about Christmas with him. If it does - FTG. It isn't your fault. You wanted to do the status quo. He isn't willing to compromise. So.... do what works best for you given the constraints.
And really - I think you will be surprised at how fun Christmas will be if you just shift your attitude about the date on the calendar.
One final note - I would not tell the kids anything until the day before (the 23rd) if you do this. If the kids tell your X what is happening, and your X isn't happy about the idea, you might have to put up with even more crap from him. And it's really none of his d@#% business when you decide to have Christmas with the kids.
Just keep it to yourself, and on the 23rd, tell them you just got special notice from Santa that since they are going to be elsewhere on Christmas morning, he decided to make a special stop just for them on the 24th.
ETA: Just had another thought on this. In light of the kids being there both Christmas Eve and Christmas morning, has anything been agreed upon about how "Santa" will be handled? Bet your X didn't even think about that. And I also bet he just plans to have them open presents from Santa at his place. Without telling you.
If he does and you do Christmas early, he'll have to deal with it. If he doesn't, there's no harm, no foul by you having Santa come a little early. Doing it this way, he will have dug his own grave, so to speak.