Law (10th ed. 2005) Husband and Wife, ?? 198, p. 270; see also In re Marriage of Van Sickle, supra, 68 Cal.App.3d at p. 740.) Section 3603 provides that a support order may be modified at any time except as to an amount that accrued before the date of the filing of the notice of motion or order to show cause to modify or terminate. (?? 3603, italics added.) Section 3603 does not contemplate that