sometimes, re-heat treating a part will ruin it.
at work we heat treated a few parts we made in house, and if not done correctly, trying to redo them usually ended up causing the parts to be scrapped because they became very brittle.
as i was not personally involved in the heat treat process, other than machining the parts, i don't know if it was the process used, the guy doing it, or other factors.
just my [very limited] personal experience. your mileage will vary.