Originally Posted by
Lusavari
There's a difference between Spells and Abilities. So you could have Fira and Second Wind II. You don't always have to keep all these crazy naming rules the same across the board. After all, Jecht had the "Sublimely Magnificent Jecht Shot Mark III" (AKA the Jecht Shot) and Tidus learned Jecht Shot II, but the magic in X followed traditional naming rules like Haste and Hastaga.
As for replacing old skills, in the field of abilities I have to agree. I'm not gonna ever use Skull Sunder again when Skull Sunder II does the same stuff place can incapacitate. The only way I could really see a need to keep the old skills is if they add some NMs that get more brutal after they get incapacitated. In other words, you'd need a skill that didn't incapacitate'em.