Just had a look on a couple of Photography forums, and from what's being said, it seems that the non-DG version has a slightly different coding on the chip, which can lead to err99 lens-camera incompatibility with certain models of digital camera. All the sites I have looked at say there is no optical difference between the DG and the non-DG version.
Looks like its a case of, if the non-DG works from the word go on your particular camera body, then to all intents and purposes you've got everything you would have with a DG version.
So with a six month warranty, any such problem should have shown up long before then. Looks like you're sorted.
Regards
Mike.