Technically yes, but YMMV. As a 2P/1P I got things changed/rerouted after schedule changes of less than 90 minutes, but I wasn't able to do it every time. I think the key is to have a plausible reason why you need the routing the way it is and why the reroute is no good. For example, if a schedule change forces you to leave, say, 30 minutes early, you could argue that you simply can't make it to the airport any earlier than the original schedule because of whatever. If it's believable, the agent is decent, and the SD takes pity (unauthorized changes will have to be made by a SD), you might get lucky. Keep in mind, though, that this route has its drawbacks: Once a SD gets involved, your record is notated, so you may not be able to just "call back" and start over. If a SD says no, s/he might mark "no" on the record, and I've never been able to override a SD's "no" decision on the PNR. Again, the key is to do your homework ahead of time -- think about all the possible objections to your excuse. What would you say if you were in the SD's shoes?