After a custody determination has been made and the case is closed the playing field is no longer level for the parents. The custodial parent now has the high ground. If the non-custodial parent wishes to modify custody he/she must now meet a more rigid standard than the "best interests" standard. The non-custodial parent must now meet the McLendon standard. He/She must be able to show not only that it is in the best interest of the child for custody to transfer but that there has been a material change in circumstances and that the benefits of changing custody outweighs the harm that would come from uprooting the child from their current home and social environment. This standard is very difficult to satisfy. Recent cases have softened the requirement regarding the weight given to the harm of uprooting the child but the "material change in circumstances" requirement remains quite rigid. The material change in circumstances requirement is difficult to prove. For instance, in a case where children were removed from a parent based on a parent's drug addiction full rehabilitation from the same was not sufficient to show a material change in circumstances. Being better able to care for your child is not enough to satisfy this higher standard. Further, if a parent loses custody to a non-parent and a case closes that parent also loses the Terry presumption discussed above.
The point is that a parent must fight tooth and nail to maintain custody of their children. If they can help it they should not allow a case to close with any thing less than true joint physical custody. Anything less will cause that parent to have to fight an uphill battle to regain custody.