You will need to file a petition to modify the original order of custody in your situation, I have outlined the procedure below.
Grounds to Modify the Custody/Visitation Order
Any person who is affected by the order can ask the court to modify or enforce the order. Before the court can grant a modification, it must find:
1. that the modification is in the best interest of the child;
2. that the circumstances of the child, a conservator or a party affected by the order have
materially and substantially changed since the order was signed by the court.
3. that a child over 12 years of age has filed with the court a written preference for a different primary managing conservator.
4. that the primary managing conservator has voluntarily given up care and possession of the child to another person for at least 6 months.
If the change is requested within 12 months of the original order, you must also show one of the following:
1. The child's present environment may endanger the child's physical health or significantly impair the child's emotional development;
2. The person entitled to establish the child's primary residence is consenting to or bringing the motion for the best interest of the child;
3. The person entitled to establish the child's primary residence has voluntarily given care and possession to another person for at least 6 months and a change would be in the child's best interest.
What is a Material and Substantial Change?
The courts have identified several events that amount to a material and substantial change. Marriage to another person can be a material and substantial change. A change in residence, age, medical condition, employment, criminal history or the relationship between the parents making the current orders unworkable can be found by the court to be a material and substantial change.
The Modification Process
The process to modify a current court order affecting custody, visitation or support begins with a petition to the court asking for the modification. You will probably need the assistance of a lawyer to file a proper petition because there are several things that must be in the petition and you will need to make sure that the petition is properly served on the other party. Once the petition is filed and served, the court can enter temporary orders if properly requested. If the modification is agreed to by the other party, the process can be completed relatively quick. If the other party wants to contest the modification then the court will have to schedule the case for trial.
There are some specific nuances to changing custody and I would encourage you to seek the assistance of legal counsel to assist you. legal Aid can offer assistance and the state bar can also refer you to lawyers who might be willing to assist you pro bono or at a reduced rate.
I hope this helps. Pete