Each item that can be enchanted has a fixed number of enchantment points. In order to apply the best enchantment, you have to beat some bosses late in the game.

 

So lets say you want to upgrade a weapon to superb (6 enchantment points needed):

- Does it mean you got to have 6 free points when you do the enchantment?

- Can you upgrade it to exceptional (4 points) and have 2 free points left and then you can upgrade to superb?

- Can you have an exceptional weapon with 4 free points, then you change the enchantment to fine (2points, 6 free points) and change then to superb?

- Can you remove an enchantment completely?

 

If you must have X free points (6 in my example) in the moment you enchant, it means when you find a useful weapon with some unique abilities (e.g. prone on crit), you have to keep it in a "bad" state until the end of the game, when you can finally enchant it to the best level.

You can overwrite fine with exeptional, superb or legendary. Same with exeptional -> superb, legendary and so on. You don't need to have 6 free points to put superb on an Item that already is exeptional. The 4 points from exeptional will be freed up and be used by the superb enchantment. So if a weapon is exeptional (needs 4) and you have 2 points left you can put superb (needs 6) on it.