BA are testing customers loyalty to the limit in cases like this.
I read of a similar case recently. I believe it was Jeddah on that occasion.
I don’t know what a business class ticket to Almaty costs, but it has to be north of £1k so to find that the plane has changed to short haul config with what is effectively an economy seat is disgraceful.
Surely BA could have sent a 777 instead?
Sadly the days of common sense and customer service are long gone and the only thing that matters is the bottom line.
Personally I would demand a refund between the cost of the CW ticket and an economy ticket because business class simply does not exist on short haul any more.
They won’t pay of course, and will insult you with an offer of Avios as compensation, in which case I would let the courts decide.
Let us know how you get on.