In the US we have National Guard and Reserve units who take hand me down equipment from the Federal services to use and keep in case of future conflict. If I recall correctly, the UK has (or had) a similar force in the Territorial Army, but I could be mistaken.
Granted, the US Navy Reserves do not operate large vessels. Still it might be worth a look at things like the Artemis, Tethys and Olympus as candidates for "Semi-retirement".
Despite photos to the contrary, I'm not sure Crusade Era ships should be included in this. I'm not sure even admitting the Third Age Raid level ships would be useful, but, if the Raiders can afford a Nova, why can't Mars? Raise everything one priority level, then allow no more than 1 Battle level FAP devoted to the Independent Mars fleet.