A Note on Personality Modification: (From the Canterlot Magical Archives)

There are several spells out there in the world that purport to be able to reform a personality, to remove undesirable traits within the target and impress upon them a new way of thinking. Care must be taken in the application of this magic, as the mind is a very complicated thing, which nopony has ever truly figured out. While a sheer volume of magical influence may create convincing short-term effects, long-lasting change will always take time, no matter the strength of the pony casting the spell. Over the course of this time, it is very important that the target be given ample positive reinforcement in their environment, so that they can learn the error of their original ways.

Without such reinforcement, the risk of the original personality relapsing is high. Even with it, a sudden change of circumstance that encourages the original behavioural pattern can lead to a full relapse. If the target was an enemy of yours, we advise that you use utmost caution.