I will be upgrading from Neoscene (v4) to NeoHD v5 soon. I was wondering if there is anything changed with the codec itself that will require reconverting source files that were converted in Neoscene in order to work with the NeoHD/CS5 setup. Is it required, and if not, is there a benefit to converting anyways?

No need to reconvert. Only exception might be if you have Canon HDLSR footage and you want all the extra metadata.