it depends on how the arrears is reported to c'link/FAO from CSA

Child support only impacts your FTB in the year in which you receive it - so if he has a debt raised against him for previous years when he was shirking his responsibilities. the assessment changing now will potentially reduce your ftb til the end of the financial and if he hasn't pad the arrears amount, it will reconcile on what he actually has paid.

it's very difficult to say what the impact will be at reco though - it's really hard to guesstimate in advance