Level 1 : Requires Coin and Wood
Level 2 (Reinforced Ship's Plank; available @1000 merchants): Requires Coin, Wood and Iron
Level 3 (available @1000 imperials): Requires Coin, Wood, Iron and Copper
You can upgrade planks to higher tiers, but you cannot downgrade them to lower tiers. Also, shipbuilding requires planks and sailcloths of the same tier.