In order to be granted a modification of custody, you must be able to prove to the court that there has been a substantial change in circumstances which will significantly affect the child's stability. Things that qualify as substantial changes in circumstance include geographical moves that would impact a child's stability and changes in lifestyle (such as substance abuse or neglect). Either the custodial or the non custodial parentThe parent that retains physical custody of the child. can file for modification of custody if they can prove it is in the best interest of the childThis is the legal standard that the Texas court system uses to make decisions related to children. When a Texas judge is faced with a decision about a child, they are going to ask himself or herself, "Is this in the best interest of the child?" before he or she makes a decision.. Proof of a substantial change in circumstance is not, however, required in requests for temporary modifications.