Can an airline rewrite a flight's origin airport for the day, following an earlier diversion?
No.  They've sold tickets for people to get from A to B. They've not sold tickets to anybody looking to get from C to B.
The aircraft would be ferried into position empty, then they'd operate the schedule that has been advertised &amp; sold.
The scenario as presented doesn't really work, though... you don't get to just strand your passengers in C who bought tickets to A &amp; tell them &quot;sorry, good luck.&quot; Either you bus them the rest of the way, or you use AN aircraft and A crew to get them where they need to be. Whether it's the same aircraft and/or the same crew as originally planned depends on various things, but as a matter of flight numbers, you fly what you've been selling tickets for (as much of that schedule as you can) and you ferry planes &amp; crews into position when you have to.
But rewriting things so that &quot;flight #X now starts in C instead of A, today&quot; doesn't happen.