Depending on the day of the week, there are one or two flights between CMB and BKK, with different flight times.
The "sensible" option for you only operates on Mondays, Wednesdays and Saturdays (and only up to end of March), with the exception of March 13, which I'm guessing is the date of your travel.
I suppose that for some reason they had to cancel that, and they then tried to rebook you, but apparently utterly failed. Either this is a manual error, or some automated process is very broken, but that doesn't change the fact that this is just not going to happen.
If you are indeed travelling on that date (i.e. departing AUH on March 12), then the options are quite limited:
- Move your departure from AUH to March 10 (!), and you then have the original timing available, and arrive on March 11
- Nothing available on March 11
- Keeping the original March 12 departure, in which case the only option on Sri Lankan is the 20h+ connection in CMB (arriving on March 14)
- Ditto on March 13 (20h+ connection in CMB, arriving on March 15)
- Ditto on March 14
- Move your departure from AUH to March 15 (!), original timing, arriving on March 16
Note that when an airline cancels or significantly changes the schedule of a flight, you are not bound to accept what they tell you, it's only a proposal, which is usually the "best" alternative they could find. They utterly failed here, but even if they did suggest the 20h+ connection, it's not binding for you.
While the details may vary depending on the airline, the fare and its associated rules, your status, and the countries involved, you usually have the following options:
- Accept what they give you (obviously here they would have to adjust things to give you something which actually works!)
- Request a change to an earlier or later date on the same routing/airline
Those two are nearly certain. Then, they may accept:
- To reroute you on a different route on the same airline (but there aren't any available here)
- To reroute you on partner airlines (here Sri Lankan is a One World alliance member, like Qatar airlines, so you could ask to be rerouted on one of the many Qatar Airlines flights via Doha)
- To reroute you on a other airlines (many options with Indigo, Gulf Air, Etihad, Vistara, and more...)
- To refund you
The fact that you have a business class ticket makes it more likely they would be inclined to rebook you on a different airline, even more so if you have FF status with them or with OneWorld.
Now the issue is that you booked via Expedia, so you can't talk to the airline directly, and the intermediary in the middle is likely to complicate things. You should probably list your preferred options by order of preference, and submit them to Expedia, which will then submit them to SriLankan. You may give the list all at once, or start with what you want most, and if they don't accept, give them alternatives. But stand your ground: they are the ones who reneged on their commitment, they are the ones who should address it sensibly (and I agree with you, a 20h+ connection is not sensible).
Edit now we know the actual date
This is a lot less favourable: the CMB-BKK flight you want just isn’t scheduled at all (any longer?) past the end of March.
While there could be further changes to the schedule and they could reintroduce flights which suit you, at this time I wouldn’t count on it.
So your only options are getting them to rebook you on a partner airline or giving you a refund.
If your fare allows it, getting a refund via the usual process and rebooking with an another airline is probably the simplest option.
Even if the fare doesn’t originally allow it, the fact that they are the ones who changed (plus business class, plus status) puts you in a strong position to negotiate, but YMMV greatly.