As a Numerologist, I make a distinction between "organic" name changes, and "artificial" name changes.
Organic name changes are the result of events that are a natural part of a one's growth to becoming their True Self. This might include marriage, or changing one's name for artistic reasons for greater success (especially in the entertainment world). In that case, a name change is a natural next step in the artist's life. An organic name change can also be the result of an individual’s realization that the current name doesn't fit his/her personality – or the name might be connected to a disliked parent or distant relative.
Artificial name changes, however, are names designed in the (almost always) futile attempt to add or remove certain attributes without going through the normal process of learning, overcoming obstacles, and growth. I hear about this a great deal, unfortunately. There are some so-called "numerologists" who suggest adding numerical vibrations to names in order to become more financially successful, or perhaps more famous. Unfortunately, this often backfires, and the person experiences the opposite effect (the person loses money, or gets involved in a terrible lawsuit, for example). I've seen this happen far too many times. There are no shortcuts to growth and happiness.
What does numerology say about a person who goes back to their pre-relationship name? There's nothing wrong with that. The name that you intuitively feel drawn to using after the dissolution of a relationship is going to be in harmony with who you are.
When it comes to getting a Numerology Chart created, the decision of which last name to use is entirely yours. If you're undecided, I would advise to have both last names analyzed, and a combination with both names included.