I'm alittle too new on SHARES but I'll speculate..........

Your reservation has been changed and the ticket has been exchanged, but the new segment still has to be re-synced with the old segment - In other words - matched up or "adjusted" - "commanding" the computer to recognize the ticket number for the new segment versus the old segment.

Sorry, I am not computer literate in the true sense of the word. Your PNR
might look like this on the surface:

UA 123 10APR FRAIAH (original departure time)
UA 345 10APR IAHLAS (new flight/new time)

The electronic ticket (not visible to you) would look like this

UA 123 10APR FRAIAH
UA 789 10APR IAHLAS (old flight)

Yet a third screen (not visible to you) would show the actual ticket number(s) to be adjusted.

So, UA 789 ticket number has to be "adjusted" or "synced" to now apply to UA 345. This may not have been done yet, thereby causing the response you are getting. I can see your concern but I wouldn't worry about it. You can either call again and have someone do it or wait for check-in at the airport (not suggested).