Constructing a transport ship: 100w
Converting a transport ship to trade ships: 100w 100g
Constructing a trade ship: 300w 100g
So overall, converting a transport ship to a trade ship costs 100w less than simply constructing a trade ship
Similarly, converting a demolition ship or an attack ship to a trade ship costs 100w less and converting a fishing boat to a trade ship costs 75w less than constructing a trade ship.
Link to waddling duck’s video that also covers bugs with upgrades: AoE4 - Lodya Ship Guide - Strongest Naval/Water Unit - YouTube
After getting the armored hull upgrade, newly constructed attack ships seem to get a 1.5x bonus HP multiplier. This bonus HP transfers when an attack ship converts to another type of ship.
Also, when converting a ship to an attack ship, blacksmith upgrades, armored hull upgrade, and incendiary arrows upgrade do not affect the converted attack ship.