Good question.
I've never tried doing that specifically with jewels, but have tried to imbue additional properties on armor that had less than 5 mods. Doesn't work in some cases because despite having fewer than 5 mods, I believe the item had exceeded the maximum imbuable intensity for the piece. For jewels, you certainly can't add anything new if it already has 5 mods or more, but not sure about swapping already existing skill points to a different skill within the group. I suspect you'd run into another max intensity issue when dealing with the uber jewels; otherwise, you'd see a lot more useful 45+ skill jewels.