I'd keep trying until you get a favorable response. This is a situation in which they should accommodate you. I suppose you could always try "well, we don't sleep well on planes but if you upgrade us we might sleep better," though that'll probably never get anything but a laugh
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I think by explaining to them that you can't miss a day of work, but have to be in London that morning for XXX very important event which their schedule change is causing you to miss may have the best results. Is the issue that award tickets are no longer available/not available from DSM?