I am pretty sure it doesn't upgrade the missile attacks or artillery attacks. Just the crews and archers melee attacks. xD And yeah if you are re-training troops to get weapon/armour upgrades you need to be able to produce that unit there. That's why the Spanish peninsula Rocks! You get 3 iron provinces, which I use to dedicate one for swordsmen, one for cavalry and one for the rest.

PS - \m/ West Yorkshire ownz \m/