Been wondering about this for a while. There seem to be two different systems for allocating callsigns to delayed flights and I can't figure out what drives them. Some flights just get a letter after the original flight number e.g. BA9X for the delayed LHR-BKK on Nov 12th and some get a four digit call sign unrelated to the delayed flight number e.g. BA272 becoming BA9608 on Nov 7th. I did wonder if it was something to do with whether the flight had been diverted, but seemingly not the case as the delayed BA68 on 28th Oct became BA9605...

I'm sure some bright person out there knows......