The latest press seems to indicate we aren't converting Frigates/Destroyers into corvettes. From the sources published, if the Hunters are cut back its for something bigger and more capable.
OPV's
Instead we are possibly converting surveying/MCM onto OPV's, and thus freeing up crew for small batch of OPV with some level of self protection.
Regarding the OPV's I think it will turn out something like this.
6 OPV80's with a 25mm gun and nothing else. These are the first batches that are currently already built.
6 OPV80's with a slightly more capable fitout. Probably better able to support the MCM/Surveying roles.
6 OPV90's built with 76mm 8 VLS(32 ESSM), 4xNSM, 9LV, giraffe or Ceafar. They can launch towed array, or UUV, or UAV etc. Your up gun OPV.
That would be 18 OPV's. Given that we were looking at ~20 OCV's, or 18 OPV's, this would seem to be a small variation of the plan. This replaced existing, less capable ships like the Adelaide class patrol boat (300t), Huons (700t), Leeuwin class survey (2000t), Paluma class hydrographic (300t)
While there are two hull designs the opv80 and the opv90 would for most purposes, be basically the same. Its a ~10m larger. Same builder, same tooling, same materials, same contractors, same logistics, same contracts etc. Quick, cheap, easy. If in 10 years we decide we want to replace the 6 OPV's with something more capable, like a type 31, I am sure, we could dispose of 6 OPV's, that is exactly the type of ship our regional friends could operate. We then go build new ships. You could even take all the weapons off them, and they would be great OPV's for EEZ stuff. I don't really see why this is an issue at all. We budgeted, costed, started to build this. Going a bit more combatant, than dedicated mine hunter, is honestly, not a major deal breaker or change, and either way, you are probably going to use the OPV90 hull either way. Its fine, move on.
Destroyers
What is definitely up in the air is that Australia needs more capable destroyers. We have 220 TLAM, that is a capability, we have identified, and purchased and are procuring. While not able to take out mainland China, it would be enough to take out any Chinese base in our region that was to be established in Fiji/Vanuatu/PNG/Solomons/Tonga/NZ etc. As such, it no longer makes any real sense for China to seek a base in this region. It would also give pause before sending a small Chinese fleet in to annoy the region.
But we need a platform that is able to fire the dam things. We probably want SM-3 capability, for much the same reason, deter deployment of missiles within our region. The Hobarts have no growth capability, they have no endurance improvement capability, they have an old radar and no power improvement capability. And we only built 3 of the bloody things and we are going to take all of them out of the water.
Eventually we may be able to fire them from a SSN, but even then that 3 boat capability isn't going to happen until ~2040+. At the same time we are going to take our Collins boats out of the water. Hassling the Americans to get subs earlier isn't a strategy that will get you anywhere, they can't. Even then, launching TLAM from SSN isn't ideal, against a peer with significant sub capabilities. It is a waste of a very valuable SSN. A SSN by itself doesn't have the land attack load out capability.
So, from what I see, reading the tea leaves, basically Australia is looking at 3 proper, destroyers, as fast as possible. To pay for this, is cutting hunter from 9 to 6. So be it. But proper destroyers. Trading 1 ship with 32 vls for another with 48 probably or may not meet the capability requirement. Being the rule of 3's, the big destroyer would probably carry something like 48-64+ TLAM, while a Hobart could carry something like 8-16 in combination to its air defence role. You will have only one of each deployed usually at a time. So a total deterrent of ~ 56-96 TLAM. Decent.
From the American point of view, something like 3 burkes (capability, not those specific ships, capability not platform remember) it what it would mean. It is pointless to worry about hull numbers, or doing it with out costing money, or getting something smaller. This is the capability Australia needs. Just build it. IMO forget tier 2, forget corvettes, forget everything else. We need a class of ships to fire the missiles we have and will procure. We have the yards, we have the talent, we have money, we have the technology. What we lack is the will and the focus.
What is important is capability. Not platforms, not hull numbers. You have things, but gain no overall capability. Replacing An Anzac with 2 OPV's does nothing for firing TLAMs and deterring a major peer from establishing bases.
Sitting on this until 2024 helps no one. The world is on fire. Thousands are being killed. The Wests armoury is empty. The sky is falling. Not making a decision is going to cause regional instability and uncertainty. Companies are going bust. People understand, we can see what is happening, countries are literally being invaded. We need to get the wheels moving on this right now.
We need to prioritise the capabilities we need. And do it with urgency and purpose. Not have review after review after review, then keep the results secret, until the opportune time politically. The time is now.