If a person learns a piece, and they grown and expand as a musician as a result of learning a piece, than the integrity of the music is not being compromised. As musicians we can learn just as much from a bad performance as a good performance. And there is no rule saying that a person can't return to a piece that they learned previously (perhaps before they were technically and musically ready to play it) and approach it again with a higher level of musicianship. The important thing is to learn from the music and to become a better musician.