I've only been able to switch between LGA and EWR in irregular ops situations.

I think it's important for everyone to distinguish between their experiences in three situations:
1. Before you get to the airport.
2. At the airport in a standby situation that is not an irregular op.
3. Irregular ops.

I think almost everything is up for grabs in an irregular op - both due to weather and due to a non-weather related delay.

Just for info, I've had problems trying to deal with an irregular op over the phone. At the time (and I think it's the same now), there are somethings that phone agents are just not empowered to do that they *can* do at the airport.