It would cost a lot but keeping a ship operating while giving the crew some down time would keep it running, up to date and us ready to rumble on or deployments. MM1(SS) Gold Crew SSBN 640.
I’m sure this is one of those sixes or sevens types of question. Operating the ship hard usually means eventually breaking it hard. Or do you put it into storage and operate it just frequently enough to keep it updated and ready for immediate activation?
The underlying issue is shifting from peacetime tempo to wartime tempo. Just how much of that 355 ship goal is necessary to permit a reasonable (e.g. sustainable) peacetime operational tempo? 300 ships? 310? 320? How will the Navy flow into the war zone as the surge hulls manned by USNR crews come on line?
A key point to note in the CBO report is that only 5 shipyards in the United States are capable of building major combatants. Even they will require significant investment ($4 billion) to meet the demands of the 355 ship fleet building program despite it being carried out over a nearly three decade time frame.
The notion that there is some significant excess naval combatant ship building capacity that could be brought on line quickly after the emergency is upon us is not realistic. Heck, the Navy identified that bottleneck in production prior to WWII (and those were much simpler ships to build).
Given the time, anything is possible. But the fleet as it exists at the beginning of the conflict will have to carry the load until any new production capability begins deliveries. The time to build up the reserve is before the event even if you have to store it to contain peacetime O&S costs. Who knows? The cost savings may even be enough to offset a significant fraction of the shipbuilding budget increase.