It's best to transfer it to buffer and refrigerate it.  Better still, treat it with osmium before storage. 

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.

Thanks both of you!

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.