A person who has sinned for ten, twenty, or fifty years, feels the immense vacuum in that part of his life.
Although he strives desperately to fill it, there seems to be an emptiness that nothing can satisfy, and his past evildoing, even if thoroughly repented, becomes part of the structure of his soul in the opposite direction.
This is why it is said that, in a way, a true penitent, with that extra power of recollected sin, stands on a higher level than a Tzadik who has never sinned at all.
The nature of the transformation is more like a chemical change, a change of essence rather than a change of form or place.
Everything that was true of a person is transmuted into a different substance.
–Rabbi Adin Steinsaltz