So, what I'd like to do is add an ability to upgrade a ship where you can choose which ship design (of the same hull type) to upgrade to, as in this idea: Allow ships to be upgraded to any design of the same hull type. I looked at EntityActions[Fleet].xml and upgrade is not listed (I guess since it is a ship action, not a fleet action). Is there any concievable way this could be done?

If, say, I would have to jury rig some solution (like by creating a custom "quest" that gives you a choice of ship designs to use): is it possible to replace a given ship that is already built with a different ship in response to a button press?

While we are at it: is there any way to mod the Vodyani Ark hull to allow a new ship design using the Ark hull (to have more than one Ark design at a time)?

Hey eruanion!

Unfortunately I think what you want to do is impossible:

the upgrade action is not present in XML (its cost can be modified, but nothing else), and it's not possible to add the "code" behaviour or the required GUI (buttons, preview, stuff like that?) with a mod.

A quest could be an interesting alternative, but it would be very complicated and would require to destroy the ships and re-create them with the correct ship design afterwards, since there is no action which allows to replace a ship's design or upgrade it.

Thanks a lot for your answers!