How high would the probability of equipment change for HKG-BKK? (e.g. regional to flat bed, or vice versa)

And when would they usually be changed?

Nothing is guaranteed on regional flights. Iv had 3-4 changes of plane type when flying to/from BKK. Sorry!

"Higher than you'd prefer". And can happen at any time: months in advance, multiple times before flight, up to an hour before the flight.

This is probably the most-posted topic in the forum! You can refer to these (ominous) threads:
https://www.flyertalk.com/forum/cath...ue-ticket.html
https://www.flyertalk.com/forum/cath...f-bkk-hkg.html
https://www.flyertalk.com/forum/cath...ent-swaps.html
https://www.flyertalk.com/forum/cath...ess-class.html
https://www.flyertalk.com/forum/cath...ngle-seat.html
https://www.flyertalk.com/forum/cath...-question.html
https://www.flyertalk.com/forum/cath...st-minute.html

There are many more threads, but those are some of the recent/longer ones when searching for "equipment swap". Searching always appreciated!

Some flights and routes are better protected and/or have higher odds for various reasons, but none are totally protected except PEK (for now) on CX390/391, and HND on 548/549. That's because these two flights sell and protect regional F class and hence longhaul J is also guaranteed. For those unfamiliar with CX, it is better to just book regional flights for your schedule not equipment. I've read some blog posts implying it's possible to secure longhaul equipment on regional routes, which is deceptive, although it's easy to see how it fools infrequent CX customers. Because the equipment is loaded in the system, on EF, etc. But just bear in mind CX frequently and without any qualms swap it away at any time. Except the PEK and HND flights I mention above.