Many Earth magic spells are reversible (for example, Destroy Undead and Create Undead). These spells do not have to be learned or “memorized” separately. In fact, there is no way to separate the two spells. If you know the spell Cure Wounds, you automatically also know Cause Wounds. You cannot “forget” how to cast necromantic spells or otherwise honestly claim to only know one side of a reversible spell.
You can decide at the moment of casting which spell effect you will use (with the proper incantation, of course).
The reverse of many healing spells are necromantic, identifiable by the word “Necromancy” in their incant. Be aware that in-game laws exist against necromancy; if you are discovered casting necromantic spells such as Create Undead, you’d better be ready to face the serious in-game consequences if caught (the penalty for casting necromancy is almost always at least one character death).
This works a little differently for Magic Items and Potions. If a reversible spell is put into a Ritual such as Enchant or Spell Store, it may only be brought back out in the same way it was put in. Similarly, once created, a Potion only holds one “side” of a reversible spell.