Under the Texas Family Code, you can modify the terms of conservatorship and/or possession and access in a prior order if at least one of the following three conditions is met: (1) the child, if at least 12 years old, tells the judge in chambers that he/she wants to change who can decide where the child lives, (2) the conservator who had the right to decide where the child lives voluntarily gave up possession and care of the child to another person for at least 6 months, or (3) the circumstances of the child, a conservator or other party affected by the order have materially and substantially changed since the prior order.
The first two conditions listed are fairly straightforward, but the majority of modifications are based on the third condition listed. So what exactly is a "material and substantial" change in circumstances? Some of the changes courts have considered "material and substantial" include: remarriage of one or both parents, conduct and abilities of a stepparent, change in number of children, meaningful change in children's ages, criminal conviction of a parent, and meaningful relocation of a party and/or child. This list is not exhaustive but merely examples. Whether your particular circumstances have changed enough to qualify as "material and substantial" is a question you and your attorney should consider.
If less than a year has passed since the prior order was signed, there are additional requirements if you want to modify who has the exclusive right to decide where the child lives. At least one of the following three situations must exist: (1) the child's present environment endangers the child's physical health or significantly impairs the child's emotional development, (2) the person who has the exclusive right to decide where the child lives wants to modify the order, or (3) the person who has the exclusive right to decide where the child lives voluntarily gave up possession and care of the child for at least 6 months. On a final note, voluntary relinquishment of the child for a period of at least six months is not enough to modify a prior order if the person who gave up possession did so because of military deployment, mobilization or duty.