Hey, I believe I might have the answer to that one.. Or actually it goes along the same principle as what Swiss has already said..
So when you import the "land_units" table do not add a prefix and the changes you make to the stats should overwrite the vanilla values.. This is an educated guess, going on what Swiss said..Import it into your MOD and don't add a prefix before this table. Then you can delete all rows of the units which you don't want. That your table overwrites the "vanilla" table, you must call it exactly like the vanilla table.
Edit: Oh and please post if it works/or doesn't work and I will update the OP..