Buying right before a new model comes out (and being unable to upgrade) is indeed a bummer. However, it is absolutely true that there were heavy rumors (including on this site) that new iMacs were imminent. And the buyer's guide also on this site indicated that the current models were at or near end-of-life. So the information was out there.
If it was Apple you asked, then, well, what do you expect. Apple *never* tells *anyone*, *period*, about what's coming or when. This is also information what was certainly out there if you looked.
But the bottom line is: you wanted your machine a month ago, and now suddenly it no longer serves your needs? I can understand having lust after the new machines, but that doesn't mean that your machine has suddenly become worthless.
And on that front, if you really want to trade for a new one, then sell your current one. You'll take only a relatively small hit on it I'd think, and that would be your modest penalty for not having done adequate research beforehand.
So, to summarize:
1) You could have known this was coming
2) You still have a good machine
3) If you want to upgrade to a new one, sell your current one.
And that's about all there is to say about that.