I just got all ofDS9 a few months ago, and I am watching it straight through for only the second time. I tried to spot when Dr. Bashir is actuallyreplaced by a changeling and I got to "In Purgatory's Shadow" and realized I completely missed it. I tried to look up when the replacement actually occured and the general concensus seems to be before "Rapture" while he was away at Meevan IV, which I don't remember hearing anything about. And if it was before "Rapture" then the changeling saves Captain Sisko's life, when he could've killed him during surgery in "Rapture" and he let the babychangeling die when he could've saved it's life in "The Begotten."
If anyone has anything that they can add, correct or explain, I'd love to hear it