This is less about asking for an option to change ships, and more about an idea of how such a thing could feasibly work if the devs were inclined to add it. A friend of mine brought this up during an intense SoT discussion, and I thought it was brilliant enough to throw here to the wolves.
The Problem:
Most of us know the problem at hand here, so skip ahead if you just want to get to the meat of the suggestion. For those unfamiliar with the problem this suggestion is trying to solve, and for general context for discussion, read on.
Sometimes you're on a sloop or a brig, and you have a friend who wants to join you mid session, but your ship is at full capacity. You could always end the session and start a new one with a bigger ship, but it can be a time consuming process with loading screens and a major loss to your supplies. Or maybe you have the opposite problem. You're on a galleon or brig and some friends have to leave, leaving your ship understaffed with the choice to either try and make it work, or start a new session while sacrificing all your supplies and emissary progress. Yes if you started a new session, you'd also lose any unsold treasure and voyage progress, but for the sake of argument lets assume we're not concerned about those things.
The Complications:
Letting players change ships mid session has been suggested a lot, and I recognize that. And while it may seem nice on paper, it does come with a lot of issues like server capacity with ship types, treasure transfer (or lack thereof), gaining an upper hand against another ship and so on. Issues both technical and logistical. But there is a solution that has been staring us right in the face. A mechanic that has been in the game for a while and has since been developed on further after its initial introduction.
The Solution:
Diving! Or the tunnels if you prefer. Be them tunnels of the damned or tunnels of the ocean floor. When you dive or return from an instanced tall tale, you get migrated into a new(ish) server which has the proper capacity for your specific ship type. So why not just use that to circumvent some of our technical hurdles? Give an option for players to change ship type, which will then prompt them to sail into the open ocean to dive for the change. Give it the exact same rules that diving (or tunneling) for anything else has. You keep supplies and storage crates. You lose treasure. You drop a copy of your emissary flag but keep the flag itself. You (maybe?) retain your current voyage progress (this is very hit or miss in the current game and I can't tell if its bugged one way or the other). And then you just emerge on your fresh new ship, on a fresh new server, ready for a vessel of your size to fill an available slot. It can use basically the same rules and restrictions as hourglass diving. The actual in game UI for this process can be something like, placing and voting on a "voyage" to "dive" and change ship types.
FAQ:
Q: Wouldn't this be abusable by people trying to escape a fight?
A: No, as it would use hourglass rules for diving, requiring no other ship be nearby before a dive can be made.
Q: Wouldn't this let people who gain some distance in a chase dive and escape with their treasure and flag?
A: No, their treasure would be dropped right where they dive. Their flag would leave a copy of it behind to be picked up, just like it is when sailing into the portal for tall tales.
Q: Where would your storage crates go when changing the size and shape of your hull? Wouldn't they clip into the ship or get thrown off?
A: The crates can have a designated "spawn point" on each ship where they are moved to during the transition.
Q: Couldn't someone starting a Fort of Fortune or Fort of the Damned with a sloop, who sees a reaper coming for them, then change to a brig or galleon for greater advantage in firepower and player numbers?
A: No, as initiating the change (dive) will force the crew and their ship to merge into a new server, leaving the fort they were on behind.
Q: Doesn't this idea seem redundant when you can just start a new session with a different ship anyway?
A: Maybe. However it is only as redundant as the official in game option to scuttle and change seas, less so even. When scuttling and changing seas, everything is lost (save for your voyage), making it (almost) no different from starting a new session. I believe it's meant to be a time saving mechanism, much in the way this suggestion aims to be as well. And a lot of players, myself included, can spend quite a bit of time building up supplies only to have sudden changes in plans with friends. It sucks to have to throw everything away to start a new session every single time that happens. This has been a frequently requested thing and this suggestion is more to show how it can be near painlessly added to the game, while avoiding many of the potential issues people might cite around the concept.
Alright, that's all I got. Feel free to rip and tear into this.
