TBH I suspect that funding a that level is not for upgrades. One needs to remember that the Arafura-class OPV, when 12 units were ordered back in 2018, had a total cost of ~AUD$3.6 bil. and a rough per ship cost (again, in 2018 dollars) of AUD$300 mil. If budget line items for the Arafura-class in the Billions are being mentioned, I think it would be more likely that more units are being considered.
If this is accurate, then perhaps any new units if ordered, might be of to different layout and configuration, a Batch 2 OPV design if one will. That would make more sense to me than Australia pouring (flushing?) hundreds of millions of dollars per vessel into a small number of new OPV's that is only just starting to enter service.