Courts can typically adjust support back to the date that a person received effective notice that the amount received needs to be re-negotiated (DBS v SRG, 2006 SCC 37 at paras 5, 124).

There's no special form to use, a text message or email can be sufficient. A discussion can even be enough, although if it's only oral it can be difficult to prove later.

However, even when a court goes back to the effective notice date, it won't go back more than 3 years before the date of formal notice. Formal notice is different than effective notice, formal notice means when someone actually filed for support in court.

Many lawyers and even judges often incorrectly confuse these terms (Smith v Gulka, 2020 ABQB 32 at paras 106, 115). You don't automatically get to go back 3 years, you're still limited by the date of effective notice, that just can't be more than 3 years prior to the date of formal notice. The idea is that the other parent needs to know that they may have to pay, but even if they know, you generally can't sit on your hands for more than 3 years doing nothing, then go for a large amount years later.

There may be other exceptions, known as the DBS factors:

1. Whether there was reasonable excuse for why support was not sought earlier. For example, did they lack the financial means, emotional means, receive inadequate legal advice, lack knowledge or information about the other parent's financial status, did they have a
   justifiable
   fear that the other parent will react vindictively to the detriment of the family, or did the other parent engage in pressure or intimidation? See DBS v SRG, 2006 SCC 37 at paras 101-103.
2. Did a parent engage in blameworthy conduct? Did they hide income increases to avoid support increases, did they intimidate or dissuade support applications, did they mislead the other parent into believing their obligations were being met when they didn't know that was the case, or otherwise make an improper conscious choice not to recalculate support (SR v TR, 2020 ABQB 251 at para 36)? For example, failing to disclose a significant increase in income may be blameworthy conduct (Goulding v Keck, 2014 ABCA 138 at para 44).
3. The circumstancs of the child. Did the child benefit financially in another way (Goulding v Keck, 2014 ABCA 138 at para 52)?
4. Any hardship occasioned. Having to pay a large amount of arrears or an overpayment may be addressed through a structured payment plan though (Goulding v Keck, 2014 ABCA 138 at para 58).

Courts might also go back further if an agreement or order required that a person exchange financial disclosure each year, but they fail to do so (Goulding v Keck, 2014 ABCA 138 at para 62). It may not even matter that the other parent didn't exchange theirs either (Johannson v Haaranen, 2019 ABCA 73 at para 20). However, in some circumstances courts might consider the DBS factors and decide not to go back that far. For example in McBean v McBean, 2019 ABCA 1 at paras 20-22, 26, support did not go back that far where the formula used in teh past was unclear, income was above $150,000 and not subject to a clear formula, the parent contributed financially in other ways such as direct payments to the adult children, the other parent unreasonably delayed their application, and information was provided by the parent when it was requested.

Court orders in regular or special chambers in the Court of Queen's Bench are often made on an Interim basis, and can often be revisited as part of the final settlement or at trial. Similarly, orders made on a "without prejudice" basis can be revisited without having to show that circumstances have changed.

The same rules factors generally also apply to overpayment of support. However, effective notice of overpayment requires clear communication of the change in circumstances, accompanied by disclosure of any available documentation necessary to substantiate the change (Colucci v Colucci, 2021 SCC 24). The other parent should be given sufficient reliable evidence to allow them to determine when and how far their income fell, and whether the change was significant, long-lasting, and not one of choice (Colucci v Colucci, 2021 SCC 2). Providing an estimate on its own might not be sufficient (Moen v Ropchan, 2021 ABQB 777 at paras 17-19). It depends on the circumstances though, for example in one decision, a parent knowing that the other had been incarcerated was sufficient (TM v ZK, 2021 ABQB 588 at para 38).

Courts won't reduce arrears just because they're very large (Stephenson v Racine, 2009 ABCA 233 at para 6). If someone is presently unable to pay, then arrears might be suspended for a limited period of time, or instalment payments may instead be set (Haisman v Haisman (1994), 157 AR 47 (CA) at para 26). Arrears might only be cancelled if they can prove that they cannot and will not ever be able to pay the arrears (Colucci v Colucci, 2021 SCC 24).