Play it as set..when I first stated gaming an age ago there were no CR's. You would be going through an adventure, sac the room full of goblins then tun into..GASP..a blue Dragon.
Not all fights are fair, not all fights are winable. Sometimes you run into things that will steamroll you.In these instances you get creative or retreat.
I say play the encounter out, it will be tough, but if they conquer it the players will feel like they accomplished something grand. If things go badly then they will retreat or die, and yes some characters may fall in battle but tha is the point isn't it. Without danger, without knowing that each battle may be your last, what is the point?