If you're interested in modifying the child custody arrangement that you have with your ex, the key is to focus on your child's best interests when you ask for the change.
The court does not want to change an existing custody order for frivolous reasons. For instance, perhaps you feel angry that your ex has started dating someone new. You decide to seek full custody of your child simply as a way of getting back at your ex for this new romantic relationship.
That's not a reason the court will respect. The court always puts the child's interests first when determining custody. The child's well-being and way of life cannot get interrupted simply because of something a parent wants.
However, there are valid reasons to seek a modification --and times that your wants will align with your child's needs. For example, maybe your ex's new partner has a criminal history and you fear for your child's safety and well-being when they're together. Perhaps your ex has been ignoring the child custody schedule that you have now due to influence from this new partner, and you want to seek full custody so that you can stop dealing with these violations. You and your child would both benefit from a change in custody orders.
When asking for a custody modifications, you have to think about how the changes will impact the child's life. If the changes will somehow serve to keep the child safe, provide a better living situation or give the child more opportunities in life, that's a good reason to request a modification of your custody agreement.
This can become a complex and contentious issue. Even when you have a valid reason, your ex may oppose the action strongly. You need to know all the legal options you have and how to officially (and effectively) ask the court to order the custody modification in order to be successful.