If a dad has 3 years left to pay child support, and he decides to retire, does the agreed upon amount necessarily change?

Retirement, by itself, does not change the child support order.  The father would need to file a motion to modify the child support order.  If he does, you may argue that he is not entitled to a modification because he is voluntarily unemployed.  If the court feels there is insufficient justification for his retirement, then the court may attribute income to him if the court feels he is voluntarily unemployed. Please let me know if you have any other questions.  Many thanks.