Superb and moderate are potencies, which are determined by the level of the enchantment, which should in turn be based on the level of the item. You can't improve an item's level, so you can't improve the potency of its enchantment-- unless it was an item that you applied a glyph to, in which case the glyph could have been of a lower level than the item.

If you mean the quality or aspect of the enchantment, I think the enchantment quality matches the quality of the item, and I think that if you improve the item's quality it automatically improves the enchantment's aspect as well-- unless the original enchantment was overwritten by applying a different enchantment to the item, in which case the enchantment's aspect could be different than the item's quality.

thnx!