Semantics aside princess, the truth is that post judgment orders changing the level of child support payments, up or down, can and have been given retrospective effect. But it is not a matter of chance or judicial caprice as you have casually indicated to the OP with your indecisive "maybe" and "you can ask" responses.
Often such orders emanated from a finding that a parent failed to disclose sources of income, or a subsequent significant increase or decrease of income. Other factors are the resultant hardship on parent or children, the needs of the children, and the timing of the motion because an unreasonable delay can often militate against the applicant.
Moreover, the effective date of the order can relate back to beyond the date of filing or service to when the applicant notified the respondent of the intention to seek modification.
In sum, there must be demonstrative showing of a change in circumstances justifying a post factum order. And since the OP has offered nothing to indicate the presence of any grounds to support such a request of the court (retrospectively or prospectively) I don't believe it appropriate to encourage the OP to take a stab at it.