I sympathise with you but the situation with CX is quite dire at the moment, LHR flights being changed around, frequency dropped from 2x daily to 1x. Bkk flight also only 1x daily and the scheduling is a mess with diff times on some days.

my word of warning given the current situation with CX, even if they do rebook you to a new CX flight (i.e on a different day) there is no guarantee they won’t just re-adjust the schedules again and again afterwards, do you want to be unnecessarily stressed every day leading to the day of departure not knowing if CX will again mess up your flights?

Flying connecting flights with CX is a bit of a Russian roulette gamble at the moment - you can try their UK call helpline (usually a 1hr waiting queue) but most likely outcome they will offer different date of travel…if you type contact us on the CX website that should bring up the UK number