just like u can change the color and engine of a car... lets say u got a black civic and u repaint to to orange and get the papers amended.. it legal.. however... lets say u got a mehran and u try to convert it into 2016 alto, it wont be possible,
u can change the body with the same year or same frame number.. lets say u got a 1995 prado,.. it falls off the cliff.. body is ruined... so instead of rebuilding it, u swap the body from the same frame and model range.. u can't install a 2014 body on 1995 frame.. however u might be able to get a 1999 body on 1995 frame