  In this work we present for the first time the comprehensive study of the
Melosh spin rotation effects in diffractive electroproduction of S-wave heavy
quarkonia off a nucleon target. Such a study has been performed within the
color dipole approach using, as an example and a reference point, two popular
parametrizations of the dipole cross section and two potentials describing the
interaction between Q and bar{Q} and entering in the Schroedinger equation
based formalism for determination of the quarkonia wave functions. We find a
strong onset of spin rotation effects in 1S charmonium photoproduction which is
obviously neglected in present calculations of corresponding cross sections.
For photoproduction of radially excited Psi'(2S) these effects are even
stronger leading to an increase of the photoproduction cross section by a
factor of 2-3 depending on the photon energy. Even in production of radially
excited Y'(2S) and Y"(3S) they can not be neglected and cause the 20-30%
enhancement of the photoproduction cross section. Finally, we predict that the
spin effects vanish gradually with photon virtuality Q^2 following universality
properties in production of different heavy quarkonia as a function of Q^2 +
M_V^2.
