History of modification

It is in my husbands custody order to have 6 weeks visitation in the summer and visitation over the Thanksgiving holiday. We have never had the children over the Thanksgiving holiday in the 8 years we’ve had the current custody order. In lieu of having them over Thanksgiving we have consistently agreed to modify the order and have them 7 weeks in the summer and they stay with the ex at Thanksgiving. We have tried reaching out to her several times via text and email over the past week (we only communicate in writing because of ongoing problems and inconsistencies) and she has neglected to respond to our request for the modification. I want to purchase airline tickets soon to avoid sky high prices for last minute tickets. I know we can’t just say we’ll have them 7 weeks and she has them at Thanksgiving because ultimately that language doesn’t exist in the court order, but how long is a reasonable amount of time to wait for a response and how many attempts to reach her are considered reasonable? Is there precedence already set over the past 8 years where I could just purchase the tickets on an assumption of agreement to modify based on history? Can our attorney draw up an agreement to modify the court order to 7 weeks in the summer and she keeps them at Thanksgiving and have her sign it or do we have to go to court and have a judge rule on the modification?