I've bought 'H' and 'B' class semi-flex fares (on domestics) before and not been allowed to change on the day, despite there being plenty of available seats. BA were only selling 'R' fares on the day, which I must say always seems to be the case. So, if 'H' and 'B' domestic fares don't seem to be available on the day, how exactly is one supposed to utilise the benefit of a semi-flex ticket? Is it all just a big con?