Apple is a business and had announced some time ago that the new model was in production and nearing release. Consumers have the right to choose when they upgrade which you did and your new computer is now "secondhand" and worth less than you paid for it although Macs that are looked after hold their value quite well. Apple would need to send it away for a "refurbish" to make it look and feel new. Phone an Apple Store and explain the situation: there might be a discretion to offer a slightly better trade-in than you see online but maybe not.
If you are affluent, maybe you can treat yourself to the new machine and forget about a loss made on the recent purchase. If money is a big consideration, console yourself that your present Mac will run better and faster on Big Sur and, even though it will be "second best", it will still be better than almost everyone else's computer.
I am looking forward to installing Big Sur on my Early 2015 MacBook Pro which I have to make do with for now.