The vast majority of transformations in EGS have technically been enchantments, which have the distinction of being reverseable even if one's only method is sitting around and waiting for the enchantment to end.
Not all spells that transform, however, do so via enchantment. Some just change a person permanently with no easy method of reversing the effect. It's sort of like the difference between typing on a computer and typing on an old typewriter. In theory, you can fix any typos created using either one, but you're going to have a much easier and automated time with one than the other.