You will only have to rebuy your ship if the powerplant has changed and you want to take advantage of that change.
Explanation:
The Powerplant of a vessel is specific to that ship/model.
It has been made buyable and tradeable etc like other equipment in most mods except for Discovery (IE like armour upgrades)
It is automatically included in the ship price when you buy a new one and is transfered over automatically.
The NEW powerplants will be on the sellable ships.
YOUR ship will not have the new one as it bought the 'old' powerplant.
Basically a good few ships will now come with a better powerplant when you buy them. If you want it, you will need to buy the ship, even if you already have the same (In all but the powerplant) one.
