  We show that even when spins and orbitals disentangle in the ground state,
spin excitations are renormalized by the local tuning of $e_g$ orbitals in
ferromagnetic planes of K$_2$CuF$_4$ and LaMnO$_3$. As a result, dressed spin
excitations (magnons) obtained within the electronic model propagate as
quasiparticles and their energy renormalization depends on momentum ${\vec k}$.
Therefore magnons in spin-orbital systems go beyond the paradigm of the
effective Heisenberg model with nearest neighbor spin exchange derived from the
ground state --- spin-orbital entanglement in excited states predicts large
magnon softening at the Brillouin zone boundary, and in case of LaMnO$_3$ the
magnon energy at the $M=(\pi,\pi)$ point may be reduced by $\sim 45$\%. In
contrast, simultaneously the stiffness constant near the Goldstone mode is
almost unaffected. We elucidate physics behind magnon renormalization in
spin-orbital systems and explain why long wavelength magnons are unrenormalized
while simultaneously energies of short wavelength magnons are reduced by
orbital fluctuations. In fact, the ${\vec k}$-dependence of the magnon energy
is modified mainly by dispersion which originates from spin exchange between
second neighbors along the cubic axes $a$ and $b$.
