You can’t use a Child Visitation Letter to change your current visitation schedule. The letter is only for planning a visit with your child or informing the other parent of a breach in your visitation agreement.
Changing your visitation schedule is a formal process set by child custody and parenting time laws and may end with a judge approving the proposed modifications. This process prevents a parent from changing the schedule without the approval of the other parent. It also limits the chances of the parents using the child or visitation as leverage during a dispute. The child's best interests always come first as far as the law is concerned.
For example, suppose your child visitation agreement says you get to visit your child once a week, and the other parent wants to change the visitation schedule to only once a month. In that case, they may not be able to make that change without a judge's approval. If you disagree with the proposed change to the schedule, this process gives you a chance to make a case for why the adjustment doesn't work for you.