What are the "valid" reasons for modifying visitation?
The non-custodial parent has now decided that she "would like more time with her child". The non-custodial parent willingly signed a modification 17 months ago and agreed to 3 overnight visits one week and 2 overnights the next with no child support ordered. She previously had 5 overnights per week. She now wants 50% custody with one week on and one week off. No other changes in circumstances, child is doing well academically, emotionally and socially under current agreement.
A visitation agreement can always be modified. Whether the modification is granted is up to the court. Usually, some change in the circumstances is the general ground for modification. But I don't know the facts of your case regarding why the non-custodial parent agreed to those particular terms and no custody. For example, was she having personal problems in her life that prevented her from sharing custody? If so, and she no longer has the personal problems, she is entitled to seek custody and/or a change in the terms of the visitation agreement. Or if she can now see her child more often, she can request more time. However, the court always considers what is in the best interest of the child when making any decisions regarding custody and visitation.