I don't think it's that complicated actually.
A few simple "rules":
- If you book an I class with advance purchase requirements, and if you want to change the first sector close to date, it can be cheaper to throw it away and make a new booking.
- Watch out for that advance purchase requirement. Always pay attention to it before you proceed. Paid service not required - ITA Matrix would tell you for free. Even a dork like me can use it.
- Change fees mean change fees. It does not include the fare difference.
- Don't be surprised at the fare difference. It's often quite a phenomenal amount