I can only provide a sample of the grounds (as there are many). The first
primary improper grounds to modify an order is "unclean hands".
That is, one party is just being difficult or unpleasant in either their
actions or conduct. Examples could be chronic lateness to exchanges, driving
too fast, not attending the children's school functions etc. An additional
improper grounds to modify a custody order is to punish the shortcomings
of the other parent. For example, one parent is a lousy driver, is not
equipped to assist the children with homework or just lacks social graces.
Finally, the last improper grounds to modify a custody order is to "give
the other parent the opportunity to take control of the children".
For example, if the children have chronic tardies and are failing in school
during one parent's watch, this alone is an improper ground to modify
the child custody orders. Now understand that just because the above illustrations
are improper grounds to modify custody, does not mean that the court cannot
modify the orders to curtail or correct the other parent's behavior
if it poses a risk to the health, safety or welfare of the children.