Subject: How would an alteration of a cell's DNA affect the cell?

In detail, say a new color-determining gene were to replace the 
previous gene in a person's right eye by how I understand gene 
therapy to operate.  It is my understanding that every cell in 
the human body is replaced every seven years.  After seven years, 
would the right eye now have a different color?  If so, what 
(other than the actual knowlege of how to effect the mechanics) 
would prevent a person from becoming an entirely different person 
through an enormous, entire-DNA code replacement?  Will it be 
possible to improve the current height of an adult human through 
such a process?  Grow out of diabetes?  Change gender?  Change 
species? 

Re: How would an alteration of a cell's DNA affect the cell?