Yes, when comparing the differences between SD and HD versions, the first thing I look for is whether the SD and HD versions are from the same master. When the same masters are used for both, the improvements are going to be incremental based on how much new detail is evident and the viewing ratio of screen size and sitting position, and even that is further determined by how detailed/soft the original elements are. Since the new SD and HD versions appear to be from the same masters, the "your mileage may vary" may be the best outlook.For my viewing experience in HD, as an example, I can see the fibers of Oz’s cashmere suit. A far cry from the public domain quality I’ve seen over the years, and even the superior SD quality of the various official compilations. It also seems a bit sharper than my new DVD versions to my untrained eye.
Is it worth the $30 upgrade? Your mileage may vary. A good up-res from a newer Blu-ray player may be just fine using DVDs.
However, when the HD versions are from new pristine masters, the differences can be huge. In the case of OH, compared to the previous DVD sets based on older public domain prints that have been round for a while, these new SD and HD versions may both be considered a major upgrade.