Agents can pretty much do whatever they want. Policy states "no changes to routing" but in practice, an agent can do it if they want to.

I missed the DTW-IND portion of DTW-IND-MSP-SNA. Ticket agents in DTW said that I could not change my routing eventhough waiting for the next DTW-IND-MSP flights would cause me to miss the last MSP-SNA flight. They said they'd be happy to collect the $1,500 fair difference to put me on the DTW-MSP flight instead though. Finally I got an agent that put me on the DTW-MSP flight and in F to boot (before I was elite) and made my original MSP-SNA connection.

Also for and ATL-DTW flight I tried to fly standby on an earlier flight, asked the GA, she said not to bother, the flight is completely full, I asked to be put on the list just in case and handed her my PE card, she then handed me a ticket in F.