Behind this question was another; does dng conversion do anything more than just change the file format? If I could do, say, .orf > dng > .orf, would the two raw files be absolutely identical? [and I'm thinking of translations between languages, where after a few iterations, the original meaning is totally garbled].
Translation from one data structure to another is always a little dicey until proven by lots of testing experience, both in the lab and in the field. Earliest versions of the DNG specification lacked the means to encode various and fluffy bits of the many native RAW files that were not considered "essential" to the RAW data and processing it, so indeed at one time there were several bits lost in the conversion. (None of them significant to processing the DNG file, mind you, since none of the manufacturers' proprietary RAW processing software will process RAW files of any format that are not generated by cameras it supports...)
The DNG file specification has grown since its inception to accommodate a lot of additional bits. Beginning with DNG Specification v1.2, all private data and maker notes are identified by the DNG Converter (and LR and CR) and transformed into well documented token-data constructs inside the DNG file. Of course, it is still the case that only the manufacturers' RAW conversion software knows what to do with many of those bits, and none of them process DNG files not made by the cameras they support ... nor is there any transform possible to take a DNG file back into the original, native RAW file from which it was generated.
What this means is that if you translated all your native RAW E-1 .ORF files into DNG format and disposed of the .ORF files with an early version of DNG Converter, some data might have been lost that some
possible DNG compatible application in the future
might have been able to use to your advantage, and there's no way to get it back. If you did the same translation with the later or current version of DNG Converter, the possibility exists that you could take advantage of this new capability. The chances of such capability coming about are small, the significance of the advantage is also small (since you have presumably converted to DNG to do your processing with something else already and will be unlikely to need to reprocess again from scratch all your work...).
To future proof your files against possible developments of the DNG specification that *might* include some new data construct or whatever which isn't currently handled well, the only option is to either embed the original native RAW file into the DNG file such that it could be extracted back into the manufacturers' proprietary software in the original, native RAW form, or reprocessed by a later version of DNG Converter into an updated DNG file. This generally doubles the size of the DNG files at least, but if you really really must have absolutely all the data that existed for some once and future moment, it's the way to go.
Of course, all the essential data required for a DNG compatible app to process DNG RAW files is there in the files already. So if you rely upon DNG files for your work, and an application that uses them, there are really no worries at all. The Open Source dcraw converter knows how to convert DNG files too, so even if Adobe dried up and blew away in the wind, source code to interpret DNG files is immortalized on the web.
I archive all my native RAW files onto a separate disk drive, have been doing so since 2003. All my working files are converted to DNG format since the introduction of Camera Raw v2.4. I have never once actually NEEDED to retrieve an original RAW file from the archives ... my only reasons for doing so, to date, have been purely for experimentation and research purposes.
