Ideally, this would be spelled out in your agreement. The orders from the court are, essentially, private contract law. All of the terms should be defined and specifics about how and when should be detailed in the plan submitted to, and approved by, the court.
However, if you guys have a specific agreement and you both are willing to follow it, then all is well, and the two of you can simply work that out.
As for changing your W2s, I would speak to an accountant on the specific tax ramifications relating to the years where you are not claiming the child.
Nothing in this communication should be considered legal advice. Nor should this communication lead you to believe that an attorney-client relationship has been created. Only after thorough analysis of your case with you in my office can I provide effective legal advice on your specific issue.