Pillars of Eternity enchanting

Solution 1:

I just tried it and you can update from fine to superb just fine. The end bonus were the ones for superb, as expected.

Solution 2:

Each weapon has an anvil limit of 14, but it also only has three available enchantments to you (not counting unique weapons that come with special enchantments you can't alter, e.g. "Marking"): Quality, Secondary Damage, and Slaying. Upgrading the quality from Fine to Superb would use up 4 available anvils, and the difference would be an extra +8 to accuracy and +30% damage.

A plain Fine sword is 2 anvils, +4 accuracy, +15% damage. A plain Superb sword is 6 anvils, +12 accuracy, +45% damage.