  We observe density induced 90$^{\circ}$ rotations of the anisotropy axes in
transport measurements at half-filled high Landau levels in the two dimensional
electron system, where stripe states are proposed ($\nu$=9/2, 11/2, etc). Using
a field effect transistor, we find the transition density to be
$2.9\times10^{11}$cm$^{-2}$ at $\nu$=9/2. Hysteresis is observed in the
vicinity of the transition. We construct a phase boundary in the filling
factor-magnetic field plane in the regime $4.4<\nu<4.6$. An in-plane magnetic
field applied along either anisotropy axis always stabilizes the low density
orientation of the stripes.
