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I agree with the above. I'd happily pay a supplement to travel in a special coach on an advertised service (and indeed have done so), but would be mightily miffed if an advertised service was suddenly unavailable because of whichever stock had been pulled out that morning.
It's a different matter if you can run a pre-booked 'extra' that slots in around the regular services, but most railways don't have the paths available to be able to do that.
With gala fares, the method I've noticed seems to be to make all of the non-gala days discounted and then suspend the discount on gala days. Nobody can really complain then!