He never said they could be redeemed by being turned back to what they were. He just said after their creation they were part of God's world and thus could be good. One of his essays says they have a irremediable allegiance to evil and would keep that even if they rebelled against Sauron (since he isn't the source of their will). This looks to be one of the things he had small contradictions in his ever changing story even if the published one stayed the same.
Tolkien also said that every living was divided during the War of the Last Alliance except for the Elves. That does imply that Orcs fought against Sauron, right? So wouldn't those fighting against Sauron be morally grey if anything?