Last week I had a 4-segment run from CAK-DTW-GRR-MSP-MCI but when I got to the airport I was really tired, so asked if there was any chance of an invol re-route. The ticketing agent rerouted me CAK-DTW-MCI, in 3A from DTW-MCI. Much nicer than 3 coach flights (I was in FC MSP-MCI already).

Initially one of the ticketing agents said I'd need to pay or use miles to reroute, since the only seat available on the DTW-MCI leg was in FC, but I pointed out that I was Silver Elite, and that did the trick. It never hurts to ask. :-)

This was a NWA ticket flying on my OP number, btw.