International gets a lot more trickier.
You need to read the full rules available for the fares and compare. Different international fares have different rules - eg some are non refundable at all, others are $100 cancel fee etc.
When it comes to changes, it's nearly as tricky.
I'm not aware of fare basis affecting any <minuscule> chance of an op-up, although some agents may use that as a deciding factor.