I think no matter what your ship type is, when you level up and go looking for a new ship the ship vendors should give you the option to upgrade your current ship to the newer ship stats, the gold cost to do so would be the same as buying the higher level ship. New equipment would then be added to the ship parts vendors that would give comparable stats at the same prices for each ship type as the equipment for the newest ships keeping the gold costs relatively the same for all players. Ship types could also be given their individual boosts, such as some might be a bit faster than others based upon type, some more powerful, some more better at turns, and their attack types and the like can be made different but comparable (in essense all ship types would be different than each other but comparable in playability and strength the same as all character classes are roughly the same in strength but each being a bit different than the other).
this is a great idea and i'm glad someone thought or it. I have four ships now ( raft, skiff, frigate, and galleon ) and got them because i needed a bigger ship, but not and i rarely use my frigate, and my raft is not something i'd be caught riding in anymore, so now i have two extra ships that i don't have any use for. I enjoy having the option but i would enjoy being able to upgrade .
The thing is, I don't see why anyone would want to spend money to get their favorite ship if the ship is only going to be good for a few levels and then not be compatible any longer. There is talk of ships being offered in crown packs which would add variety to the various ship types, but what variety is there really when you still have to switch over to the next ship type when you level up further. There should be a way to upgrade your favorite ship and ship type even if the gold cost goes way up.