There are several enchanting options for skills: Enchant, Blessed Enchant, Reverting, and Enchant Path Change.
Skill Enchant
This method of enchanting requires 1 [Giant's Superior Codex] and some amount of Adena. If the attempt fails, the enchant level is reset to zero.
Blessed Enchant
This method requires 1 Giant's Superior Codex: Mastery and 5 times the Adena of a general skill enchant. Should your attempt fail, the enchant value is retained, making this a comparatively safe method of enchanting a highly-enchanted skill.
Reversion
This option requires 1 Giant's Superior Codex: Oblivion, and takes the skill enchant down one level at a time. If used when the skill is at Lv. 1, the skill will be reset to zero.
Enchant Path Change
This feature changes the type of enchant applied to the skill. In addition to the Giant's Superior Codex: Discipline, this procedure requires 20% of the Adena needed for the next enchant level.