Trying to update unit types for inv. & assembly items. Can I do this with the item's internal id? What does the internal id map to?

Internal ID maps to Internal ID. What is the Unit Types that you are trying to update? (You can't change from Inventory Item to Assembly Item if that's what you want.)

I did the initial item load without specifying the primary unit type/primary stick unit type. Im not looking to change from Inventory to Assembly. Just add the unit type to existing item record whether it be pounds, cases, pallets bottles etc