In my opinion GGG should not allow either purchasing or upgrading legacy packs. Of course this is easy for me to say as an "old" supporter, but I also think there is some important history to the first packs - especially the closed beta ones.
People who purchased diamond supporter packs (particularly in the beginning) literally spent a thousand bucks on something which could turn out to be a complete "failure". Back then the game was still known only to few, and those who purchased the packs were not just buying fancy MTX or some item-design - they invested in something they hoped would materialize as a success. This is not to discredit the newer supporters (or packs), but I think many people fail to understand the dynamics that were at work back then.
Having said all this: I doubt it will ever happen - especially as all packs have always been sold under the promise of "this is your last chance to buy...".