I have an imperial stout in a corny keg, and I'm just not happy with how sweet it is. It's already been carbonated. Can I let it go flat, rack it to a carboy, and throw in some high-gravity or champagne yeast to dry it up?
I don't see why not. The only issue is how much lactose or dextrin you used. If it sweet due to those sugars, then you're kind of screwed because the high gravity or champagne yeast won't eat those, if it is not from these then you should be fine. The only way to get rid of the lactose or the dextrin is to pitch some brettanomyces, pediococcus or lactobacillus, which would give you a farmyard or sour flavor depending on which one you use. Who knows though, it might be delicious. Guiness sours a portion of their stout and then mixes it back in to give it it's unique flavor. I might even aerate the beer when transferring it to a carboy since it is going to referment anyway, this way the yeast will have a better environment to do their wonderful work.