Switching schools to year round

We have a divorce agreement that says that one of us gets February school vacation, and the other gets April vacation, on an alternating schedule. Each is one week long under the traditional school schedule this was designed around. We are switching our kids to a year-round school schedule where they will get 3-week vacations in October, April, and January. Which week would be considered the equivalent of the February break? Also, would April vacation be one week long or three?

You cannot simply substitute the terms of the old agreement/order for terms that fit your new situation. You need to modify your agreement/order based on the new schedule, and I would look to your current agreement/order for understanding on how to modify.