Please go to this site frim NYS and you will have many of your questions answered. Whenever circumstances change, a custodial or no-custodial parent can apply for a modification. The key will be, at some, point, whether it is being done in good faith. With regard to your two-year question, I think the site answers this as well--
"Modification of Child Support Orders
Either parent or the CSEU can file a petition in Family Court to request a modification (change) to an existing child support order. The modification petition should be based on the fact that either the custodial or noncustodial parent's circumstances have materially changed (e.g., change in income or other changes in circumstances).
Cost of Living Adjustment
Every two years the child support agency automatically reviews each child support order to determine cost of living increases. If the cost of living has increased by more than ten percent since the order was made or since the last review, the child support order amount will increase by the amount of the change in the cost of living. The cost of living adjustments can be made without going to court. For non-temporary assistance or non-safety net cases, a notice is sent to both parents when a case is eligible for a cost of living adjustment, and either parent may request the adjustment. "