Ironically, up gunned OPVs, using surplus equipment already owned by the commonwealth, could actually be cheaper to outfit than ships fitted with new intermediate calibre main guns. I am also curious as to what will happen to the surplus 25mm Typhoons if they are not to be reused on the OPVs.
I would imagine they would go in storage. I personally think the 76mm would be a fair choice for the OPV, many other nations in our region use it, it also opens up a lot of options and pushes range out a considerable distance. However, not sure how it fits onto a ship of this size. Or if the existing 76mm are even suitable.
While the most recent DWP called for 12 OPV's, the original concept was some 20 or so. Its conceivable that a future WP might call for the remainder 8 to replace various other ships that aren't getting any younger. Those could be fitted with 25mm, or the 25mm could be fitted to some future vessel as a primary or secondary weapon.
I do see upgunning as a possibility. US strategy and alliance is at best, complicated. UK global leadership (and identity) is, confused? Australia trying to project power up into the gulf(s) and Indian ocean, South China Sea, the pacific and the southern ocean. Filling some of the vacuum created by traditional powers, while managing emerging powers and being capable enough to act as a buffer or medium for relations. The complete re-write of Australia's foreign policy, a new white paper that will no doubt influence others.
Having something that allows partner nations to operate more limited ships or less experienced/confident nations underneath an Australian umbrella would seem reasonable. Australia finds herself in a different situation we thought we would be in even 2 years ago.
I would be giving serious consideration to 9VL, a new 76mm, ESSM/CIWS capability to be fitted later.
Personally I quite like the 1800 Damen Sea axe option, being potentially faster than more suitable for rough seas and flexibility. But the lurrsen would seem to be bigger and able to take more demanding upgrades and great endurance. But I don't know if the larger 2400 variants are off the table, it was originally reported that Damen put the 2400 forward, and the 2600 would seem to be an easy upsell over the 2400.
Given the current state of things, I don't think we will be going with the lowest bid on the OPV's. I don't see the situation(s) getting better long term.