Despite running for seven seasons, there are plenty of unanswered questions from Star Trek: Deep Space Nine. This is likely just how the storytellers wanted it, since the series was meant to be different from its predecessors. The biggest change was the introduction of serialization, and the larger story about war with the Dominion. Founded by Changelings who could mimic any person, Dr. Julian Bashir was the first central character to be replaced by this new enemy. But when did it happen?
Source：https://www.cbr.com/julian-bashir-changeling-star-trek-deep-space-nine/
