I just (finally) got around to reading The Sixth Watch and this was my question as well. At the end of the book, Anton says that he has been made completely human, his magical temperature increased so that he can no longer absorb power in the way Others can, but the Fuaran has proven to be capable of transforming a human into an Other by reducing their 'temperature'. The original was confirmed to have been destroyed when Kostya took it into space as the Watches, and presumably the Inquisition also, tracked both him and it until they burnt up.
In The Last Watch however, it's said that Arina was able to create an imperfect copy that was able to raise Gennady Saushkin's level to that of a Higher vampire. Knowing this, it seems perfectly plausible that she could recreate the initial spell that turns humans into Others in the first place. Even if Anton could only be raised to the 7th level, he could still regain a lot of what he'd lost- namely the effective immortality.
I think the only debatable issue with this is whether the Twilight would allow it to happen; Anton's powers were lost as a sacrifice to the Two-in-One, so it's possible that the Twilight would force him to honour that sacrifice by preventing his magical temperature from being lowered again. Alternatively, and perhaps more likely, it's possible that even if it were possible the Watches and Arina wouldn't want to risk provoking the Two-in-One into rematerializing by cheating their way out of the sacrifice.
