Custody schedule change request

My ex and I have been doing every other weekend with me picking up Friday after school and dropping back Sunday evening. Her new job is making that impossible for her some weekends and she's asking if we can do alternating weeks instead. I'm actually fine with it but want to make sure I'm not getting railroaded here. Should I go through the lawyer to modify the agreement or can we just do it on our own? I want what's best for our son but don't want to get screwed.