What you think about this?
* significant delay when switching ships (for any reason, could be because the original ship was detroyed (but crew survived) or to man a new ship or to switch to a better ship)
* discourage big ships reserve by maintenance cost
- whenever a crew is assigned to a ship it takes 3 turns before the ship become operational
- the player can set a "Military readiness level", when it's set to low then up to 20% of crew can initiate switching ships each turn (best during peace since it can cause a significant part of the military being locked in the switching state), when it's set to high up to 1% of crew can switch ships (best during war but you can't effectively upgrade to new ships since the new ships will be manned very slowly). All these numbers do not affect unassigned crew, all of them can be assigned at once.
- you pay maintenance for fleet. For active fleet you pay 100%, for reserve fleet you pay 50% but a portion of reserve fleet has a free maintenance (so the player is encouraged to keep some older ships but not too many). The player can scrap ships anytime.