We've run into this issue in our environment as well and found an acceptable workaround. We ran tried the same thing as OP (changing the value with GP, exporting/importing, both failed) but found that if you DELETE the \\UserChoice key with GP first and then rebuild it with the correct hash and ProgId keys, it works. We configured it to use item level targeting to assign the correct hash and prog id depending on if Acrobat was published to the user. Everyone else gets the Reader Hash and ProgID key.
Hope this helps.
