Backstory: CO says DH has SS6 every Christmas Eve till 9pm and BM 9pm Christmas Eve and Christmas Day - every year. DH agreed to this originally because he gave BM the house to be as minimally disruptive to his son as possible and he had to find his own place to live. He had indicated to BM that once he had his own place where he could celebrate the holiday, he would want to alternate the Christmas holiday.
Awhile ago. I asked this board how you handle the holidays and I really like the idea of splitting Christmas break - one parent gets him after school till December 26th or so and then the other parent gets him through New Years Day, alternating every year. However, BM has made it clear that she feels Christmas is "non-negotiable" and has no desire to alternate Christmas in anyway. Which is hard because that means every Christmas SS and DH, DD and I are going to be apart Christmas morning every year.
Ideally, we would like to open the CO back up and make some changes, but I was hoping to hear some recommendations of how to broach this with BM in the meantime, even if it's not formally decreed at this point. Has anyone had any experience with this? TIA!
-Edited for formatting-

Re: Reworking Christmas
Can you go for mediation for just this one thing if there is no change in circumstances and nothing else you want to modify? Although, I still don't know what it would accomplish if she won't budge... But it might get her moving a bit.
Otherwise, I don't know how old your DD is, but is it possible to just make it a family tradition to celebrate on Christmas Eve? Maybe get the kids out the house in the evening for something, and when they come back in, Santa made it to your house early! It's an awfully big world to get through in just one night, you know.
Again, just some ideas. Not sure how practical either of them are.
Other things we would want to change include we have him every other Thurs evening to Monday morning. We live 15 minutes away, so transitions are not a problem, and would really like to have 50/50 physical custody as well - we currently have joint custody when it comes to decisions impacting his welfare, but she really does what she wants anyway and doesn't give DH's decision any weight. I think the most frustrating thing is BM has stated if we went to reopen the CO, she would go for sole custody, which I really can't see how she would have a leg to stand on to get any additional time.
The frustrating thing is that any of this would likely be a mudslinging battle with a high price tag attached, and while I recognize that is the only way to really enforce it, I was curious if anyone had luck with any less standard approaches. Thanks!
Does DH now live in SS's town / school district (and MILs home was outside)? That would make a difference, if he could prove that being with DH was not disruptive to SS schedule.
Is DD SS's 1/2 sibling, or step-sibling? Spending more family time with a bio-sibling might make a difference.
Also, now that SS has started school, if your previous schedule did not account for vacations, school calendar, extra-curriculars you might be able to reopen for that.
If BM fights, it could still be expensive. You need to decide if the $ is worth it, but I would say the longer you wait, the longer that "Christmas is always with mom" is the routine, and it will be less likely to be changed.
DD is SS's half sister and he definitely loves seeing her and spending time with her.
The CO didn't account for Feb vacation or winter break around Christmas, only April vacation. BM and DH came to an agreement outside of the CO in terms of how to alternate them - there is no mention of extra-curriculars at all in the current CO.
The current CO says that BM has primary physical custody and both share custody in terms of decisions affecting SS's welfare (medical, school, etc.) That's really it - the visitation was originally EOWE and Wednesday night for dinner, but BM right after everything was finalized agreed for us to change it to Thursday night through Monday morning EOWE. The holiday schedule alternates everything annually except for Mother's Day, Father's Day (spent with the respective parent) and Christmas Eve is always with us till 9pm and then at BM's from then through Christmas Day.