I am little confused on the policy for SDC.

Scenario 1 already happened: I was trying to change my flight from a 1 PM to a 6 PM and was told several times that I had to pay the difference in fare class... is this true?

Scenario 2 upcoming: This coming Monday I have an early flight. I might finish up work in time on Sunday to take the last flight back (about 12 hours earlier). Can I do a same day change? The cost should be $50 right? Should I not check in at T-24? How can I get through security the night before my flight if I need to get to the airport for SDC?

Thanks!
NDtraveler