More information is needed. When was the last custody modification? Why did the judge change the custody from sole to joint? Also, the children are 14, did the children voice their desire to live with their father?
(Licensed in Georgia, Family Law Attorney)
If you filed for a modification of custody and the custody was changed from sole to joint custody, then you are prevented from filing another modification of custody for 2 years. Your ex-husband, however, if he was the Defendant in the first action, may bring a Modification of Custody Action if he can show that there has been a substantial change in circumstances warranting a modification of custody. The standard that the Court will use is the best interest of the child standard. In your case, it seems that he can file, because you were the filing party for the last modification and not him. However, he may have a difficult time showing a substantial change has occurred if there has not been a lot of time since the last modification action. Good luck, and please consult a local Atlanta attorney if you would like further information. Many should offer free consultations.