It depends on the reason why you are no longer working. Whether the change was voluntary or involuntary can make a difference in your case.  If the change was involuntary, such as your employer decided to reduce his workforce, or your job was eliminated in some way, you are more likely to be able to modify your support amount. Sickness can also be considered a substantial change, if it affects your ability to earn an income. Sickness might also be a substantial change if the sickness requires expensive medical care.
If the change in employment is voluntary, and you quit your job, or you were fired because of some criminal activity, you may not be able to modify your support. The court will make a finding as to whether your substantial change is allowable depending on the circumstances in your case.