I converted many of my RAW files in LR 2, prior to the DNG 1.3 Specification being released.
My questions are:
In short, there's no advantage in a straight DNG to DNG conversion. DNG 1.3 is about allowing data for lens correction to be embedded in the file; as pre-1.3 DNG files contain no such information, converting them gains you nothing.
There may however be benefit to re-converting from the original raws, if your camera happens to be one that uses lens correction in post; that would get you smaller file size, and (maybe) better corrections. If you enabled the "Embed original raw file" option (or its equivalent) when you converted to DNG in the first place, you will be able to recover the original raw file from your DNG. But that option is not enabled by default.