The Timeline of Changeling Julian Bashir
(I'll refer to real Julian by Julian, and Changeling as C-Bashir)
I often see people say or act like the switch between Julian and C-Bashir is obviously when they switch the uniforms. Because Julian is wearing the "old" one in the prison camp and he says he's been there for over a month when Garak and Worf get there.
But that timeline for the replacement is clearly not the case. Without touching the stardates and using only context clues-
The uniform switch happens between "The Ascent" and "Rapture".
In "Rapture", Sisko needs an emergency complicated unconventional brain surgery, which Julian does.
I can't believe that a Changeling magically gets all a person's skills/knowledge/memories/personality when they physically change to look like them. A newly-arrived C-Bashir wouldn't be able to do this surgery, or even come up with the idea. At best, he could quickly look up how to handle low-level medical stuff like broken bones or burns. Anything complicated like this surgery would quickly expose him. So this has to be Julian.
Another "Rapture" reason is because it would be an amazing opportunity for the Founders to want Sisko to die here. He's clearly a known obstacle for their plans. What better way than to let him die because an experimental brain surgery failed or C-Bashir doesn't come up with the surgery idea to save him? No murder or foul play needed.
"The Darkness and the Light" has Julian dealing with Kira's late-stage pregnancy and stresses. While C-Bashir would have access to Julian's notes about it, I think this is again another too complicated and unique situation for C-Bashir to handle effectively. This isn't at all a "by the book" pregnancy, and Julian has to keep a very close eye on it. Also remember that Julian's med-school specialty was pediatrics, making him even more uniquely equipped to handle this.
In "The Begotten", the baby Changeling dies. There is no way C-Bashir would let this happen. Wouldn't that break the rule of "no changeling harms another" by not doing anything to save it? He could've discreetly done something to strengthen or heal the baby, and he had opportunities to. The other Changelings have done that for Odo. This also has to be Julian.
Julian isn't seen in "For the Uniform" to make any observations.
So, if Julian was kidnapped after the uniform switch, why is he in the old uniform in the Dominion prison? Two possibilities.
1- The conference attendees were meant to wear the older uniform. In reality, the old one really isn't that old, anyway, having happened a few episodes ago. When Sisko went to Earth mid-season 4, he wore the older TNG style one the entire time, so entirely plausible for the same situation here. Then Julian was allowed to keep it to wear at the prison.
2- Julian says he was kidnapped in his sleep from the conference. "I went to bed one night and woke up here." Meaning he would've been in his pajamas. From here it's still plausible that he had the old one with him and they still let him keep and change into it.
But what I've gone with in my headcanon/writing is that C-Bashir kept the uniform(s). He'd want to be able to study and perfectly replicate it, and also have it in his quarters so it isn't missing when anyone is at his place and might notice that. C-Bashir could even actually be wearing it for the role. Julian was forced to change into an older style one at the prison that belonged to a dead officer. After all, many Starfleet officers have gone missing in the Gamma Quadrant and the Changelings might keep their uniforms for infiltration reasons. Since the uniform switch was recent, they wouldn't have any of the new style ones. And it makes sense he'd pick a blue one that was close to his size.
If you are looking at the episode stardates, the Changeling switch happens after "For the Uniform."
That is 79 DAYS between these two episodes. More than enough time for the 5 weeks needed for Julian to be kidnapped and spend that time in prison.
So there's all my evidence for this timeline question.
