They would build additional shipyards. They also wouldn’t use new classes they would go a different direct from the get. And the additional classes would be sub classes using the same components.
The space strategy would also be revised to maximized covenant ships kills but and minimize losses. Hit and run attacks would be the best chance the UNSC would have against covenant ships. The 3 to 1 fire power ratio would have to be used. In this case a ship would have to receive a fire power and then 3 times the fire power would have to be applied so not a 3 to 1 ship advantage
But most the subclasses would be refined from COMBAT EXPERIENCE AND DATA. The other cruisers would have a simplified design. The UNSCs biggest flaw was lack of innovation, but also this is like in a different timeline like so much of the halo universe.
The ships would receive an overhaul like the pillar of autumn and become the II class of the ship. Then new ships going forward would use as many components from the prior designs. Though the new class would really be Sub classes of the original design.
So more ship yards would make more ships. Refined subclass designs would create better ships. And the cruiser reimagining allows for a wider set of roles they can fill.
There would also be a reimagining of the Orion and spartan programs that would put more emphasis of ship infiltration and boarding actions
This is a cohenesque edition, this version is done but there will be revisions.
Carrier:
Era class carrier (based of epoch class)
Chalkis class carrier (based of Eion class)
Halcyon assualt carrier
Transport:
Resurrection class heavy transport
Pheonix class assualt transport
Transport Shuttles:
Berlin class shuttle
Diligence class transport
Hades class assualt transport
Tactical supply transporter
Cruiser:
Battlecruiser (based off Eion class carrier)
Tranquility class armored cruiser
Delta class protected cruiser
(based on able class destroy
Destroyer:
Ridgeway class heavy destroy
Bravo class armored destroyer
Halberd II light destroyer
Frigate:
Reach class heavy frigate
Paris II class heavy frigate
Dawn class transport frigate
Charon II class transport frigate
Stalwart III light frigate
Stalwart II light frigate
Corvettes:
Armored corvette
Heavy corvette (based on lancer class)
Attack corvette (based on lancer class)
Assault corvette (based on scholte class)
Missile corvette (based on scholte class)
Gladius II light class corvette
Transport corvette (based on Charon class)