I don't really see how the tissue would get damaged for EM by staying in karnovsky's for a few weeks. Formaldehyde is stable, glutaraldehyde a bit less, but since the tissue has been thoroughly fixed by now, some glutaraldehyde degradation wouldn't be expected to cause any change in the tissue itself.
Hopefully this is half-strength Karnovsky's you are referring too, as the full strength is too high in osmolarity. I agree with Daniel, much better to store in buffer after osmium, cold.
Of course you can-- it doesn't mean you should! Glutaraldehyde will continue to crosslink, not only hardening the tissue but also shrinking it. The reason for mincing tissue into fine