This would cause a problem if you think you hold a seat on the new flight but have not yet been ticketed.

Imagine getting to the airport and the flight is now oversold. You hold a RESERVATION which has not yet been ticketed (no ticket reissued) so you cannot be loaded onto the flight and have to be confirmed for the next flight with UA or another carrier (at best) or standby all day long if there are irregular ops like the ones we've been experiencing lately due to the severe weather in the East (worst case).

It happened to me once when checking in at DCA. The agent could not understand why she could not check me in after the Easy Chicken refused to give me my boarding pass.

She then realized that my tickets had not been reissued. She fixed it and gave me my boarding pass anyway because I was a 1K and the flights were light at the time.

Well not long after that, storms hit ORD and all flights started delaying 1, 2, 3 or more hours. Flights were canceling left and right and I knew that I could not have gotten a BP if I had tried to check in 30 minutes later.