Both of mine are on SDC issues, different trips, different agents.

1. To SDC on the second leg of a connecting flight, you have to call within 3 hours of the first leg.

Not true. It has to be within 3 hours of the flight you want to SDC to.

2. Trying the same change, different day I got this: it can't be done if you've already flown the first leg. Asked for a supervisor, came back and read this to me from the website:

No changes between origin, destination, or co-terminals

Kept telling her I was not trying to change the origin or destination, just get on an earlier connection to the same place. She insisted it meant I could not change anything between the time the trip originated and its destination. Neither the agent nor her supervisor would budge on it. Hung up, walked to the gate and got on the flight.