What to do if I want to take a lesser paying job?

You can seek a modification of child support.  Changed circumstances such as a change in income would be grounds for a modification of child support.

File an Order to Show Cause (court form) requesting a modification of child support, a supporting declaration signed under penalty of perjury stating the facts (reduced income) supporting your request for a modification of child support, and a proof of service with the court.

When you file an Order to Show Cause, the court will schedule a hearing on this issue.

Check with the court clerk to see if other forms may be required as the forms vary from state to state.

The proof of service verifies the date of mailing your documents to your ex.  You can either use a court form proof of service or you can write your own.  If you write your own proof of service, it just says that you are at least eighteen years of age and the attached documents were sent via first class mail unless stated otherwise to __________ (name and address of your ex) on ___________ (date).  You sign and date at the bottom.  The date you sign should be the same as the date of mailing and the same date you file your documents with the court. 

Once you change to the lower paying job, file a motion with the court to lower your child support obligation.  Dealing with this now will prevent issues later, including contempt actions which can result in jail time.  It will cost you a bit to get the modification moving, but it will save you a ton of headache later by preventing delinquencies in your payments.