  Let $H$ and $H'$ be two ample line bundles over a nonsingular projective
surface $X$, and $M(H)$ (resp. $M(H')$) the coarse moduli scheme of
$H$-semistable (resp. $H'$-semistable) sheaves of fixed type $(r=2,c_1,c_2)$.
In a moduli-theoretic way that comes from elementary transforms, we connect
$M(H)$ and $M(H')$ by a sequence of blowing-ups when walls separating $H$ and
$H'$ are not necessarily good. As an application, we also consider the
polarization change problem of Donaldson polynomials.
