1. Is it free to also standby on a later flight (e.g. UA and AA)?

2. If I am ticketed originally AUS-IAH-ATL with the AUS-IAH part on CO and IAH-ATL on DL, can I standby on a later AUS-IAH flight and have CO take me all the way home to ATL on their metal?

The reason I ask question 2 is because Delta has that silly $25 confirmed change business that I am trying to avoid.

Thanks for your help. Happy Holidays to all.