I assume the 'Special Longer Version' could be done in HD however, because it was sourced from original assets that probably still exist in the vaults. It'd just be a case of tracking down the relevant negatives, scanning everything and then splicing it together to match the previous SLV.
I think that all the negatives where there when they did the DE. Some scenes from the SLV turned up in the DE, some as deleted scenes. No idea if those came from the original negatives, though.