(a) Except as provided in subdivisions (c) and (d) and subject to Article 3 (commencing with Section 3680) and Sections 3552, 3587, and 4004, a support order may be modified or terminated at any time as the court determines to be necessary.

"(a) The duty of support imposed by Section 3900 continues as to an unmarried child who has attained the age of 18 years, is a full-time high school student, and who is not self-supporting, until the time the child completes the 12th grade or attains the age of 19 years, whichever occurs first.

(b) Nothing in this section limits a parent's ability to agree to provide additional support or the court's power to inquire whether an agreement to provide additional support has been made."

(a)(1) The income of the obligor parent's subsequent spouse or nonmarital partner shall not be considered when determining or modifying child support, except in an extraordinary case where excluding that income would lead to extreme and severe hardship to any child subject to the child support award, in which case the court shall also consider whether including that income would lead to extreme and severe hardship to any child supported by the obligor or by the obligor's subsequent spouse or nonmarital partner.

(2) The income of the obligee parent's subsequent spouse or nonmarital partner shall not be considered when determining or modifying child support, except in an extraordinary case where excluding that income would lead to extreme and severe hardship to any child subject to the child support award, in which case the court shall also consider whether including that income would lead to extreme and severe hardship to any child supported by the obligee or by the obligee's subsequent spouse or nonmarital partner.

(b) For purposes of this section, an extraordinary case may include a parent who voluntarily or intentionally quits work or reduces income, or who intentionally remains unemployed or underemployed and relies on a subsequent spouse's income.

(c) If any portion of the income of either parent's subsequent spouse or nonmarital partner is allowed to be considered pursuant to this section, discovery for the purposes of determining income shall be based on W2 and 1099 income tax forms, except where the court determines that application would be unjust or inappropriate.

(d) If any portion of the income of either parent's subsequent spouse or nonmarital partner is allowed to be considered pursuant to this section, the court shall allow a hardship deduction based on the minimum living expenses for one or more stepchildren of the party subject to the order.

(e) The enactment of this section constitutes cause to bring an action for modification of a child support order entered prior to the operative date of this section.