I do think that if you ask for a modification of child support that they may possibly apply the new calculations. I think there may be a limit on how often you can ask for modification similar to changes of custody. Check it out. Has your employment changed if so it may be worth it. I have no idea about the underemployed. A good resource for me- the Father's resource network, I found Attorney Sean Linnan very helpful. The courts should provide you with the number of the father's resource network.