Greater heal is currently only available for the priestess form, which is a dead end form. So the skill technically cannot be obtained if you want to progress to the second area.
So Greater Heal is obtained from the 3F scene, and the irreversible transformation does not happen until the 4F scene, so if you use a Dispel Potion after getting the spell but before the transformation is complete, the spell stays in your list. However, the spell is removed from your available spell list if you are transformed into a monster girl in battle (changing using items outside of battle is fine, as is being changed from boy to girl in battle). The reason the Greater Heal spell is always removed before entering the second area is because the first area's boss fight contains a forced transformation.
If you avoid starting the hero quest until after you become human again in the second area, you actually CAN take Greater Heal into the second area.
