Change Itinerary and No New eTicket

When changing an itinerary, I have a habit of checking the receipt online to confirm that it is properly ticketed with the new flights. I've always noticed that when it was processed properly, the receipt shows the new flights AND a new eTicket number.

Now, I recently changed an award itinerary - after the first flight was flown. And I notice the changes are reflected on the receipt, but the eTicket number did not change.

I'm curious if anyone happens to know how the ticketing system works...
- Is this itinerary okay with the old ticket number?
- Is it the case that the ticket number can't be changed after travel starts?
- And if the system is capable of changing flights without generating a new eTicket, why does it bother generating a new eTicket when changed prior to the start of travel?

Thanks!