I just attempted something similar on Monday afternoon, trying to get PHL-BOS while BOS was on a security ground hold. I wanted to get on standby for something going PHL-PVD/MAN without losing my confirmed (FC upgrade) seat on the original flight, just to have a solid contingency plan.

The first rep I talked to said it couldn't be done. She could move me to another flight, but since there were seats available I couldn't go standby, only confirmed, and moving me would remove me from my original flight. I tried again with another rep about 15 minutes later, and she was able to confirm me on two flights at once. She said that could only be done with supervisor access.

BOS ended up reopening and my original PHL-BOS flight went through fine thankfully. Not sure if the travel advisory they issued for BOS gave the agent(s) a little more leeway than usual with routing options.