The aim of this paper is to show that if an order preserving bijective
transformation of the Hilbert space effect algebra also preserves the
probability with respect to a fixed pair of mixed states, then it is an
ortho-order automorphism. A similar result for the orthomodular lattice of all
sharp effects (i.e., projections) is also presented.Comment: 12 pages, to appear in Rep. Math. Phy