I sometimes play with a ton of people at a time in an alliance. Sometimes completely friend controlled, sometimes mixed control with randoms. Often times we play for long hours, and eventually people need to drop to sleep or other reasons which leaves the crew on their ships, or mine, to manage a ship the can be bit taxing to handle with less help.
At other times we find ourselves with a few of our friends being left out of the fun due to ships being of smaller size.
I feel that a player ship should be able to be changed to a different type while maintaining the current server and alliance. Make it a shipweight option, slap a gold or doubloon price on it, and have it function by scuttling the current ship and spawning the new smaller, or bigger ship, at a nearby island.
Players wishing to preserve their old stock will simply need the foresight of using a storage crate, or two, so the stock can be offloaded, then later re loaded to the new ship manually.