  Let $H$ and $H'$ be two ample line bundles over a smooth projective surface
$X$, and $M(H)$ (resp. $M(H')$) the coarse moduli scheme of $H$-semistable
(resp. $H'$-semistable) sheaves of fixed type $(r,c_1,c_2)$. We construct a
sequence of blowing-ups which describes how $M(H)$ differs from $M(H')$ not
only when $r=2$ but also when $r$ is arbitrary. Means we here utilize are
elementary transforms and the notion of a sheaf with flag.
