  We analyse linear maps of operator algebras $\mathcal{B}_H(\mathcal{H})$
mapping the set of rank-$k$ projectors onto the set of rank-$l$ projectors
surjectively. We give a complete characterisation of such maps for prime $n =
\dim\mathcal{H}$. The solution is known for $k=l=1$ as the Wigner's theorem.
