Originally Posted by greenandgold
My Imperial Stout kid ...
I wonder if he was born a brew master?
Seriously, though, the original yeast will probably die of alcohol poisoning long before the beer is "finished." It's possible that they'll be more hearty than expected and the FG will be acceptable, but the more likely result is that you'll have a moderately alcoholic, really sweet stout prior to adding the champagne yeast. Champagne yeast is more tolerant of high alcohol content, so it will ferment long after the original strain dies out, resulting in a drier end product. I recommend following the directions because the champagne yeast will only help your beer get to where it should be. If there are no more fermentable sugars when you add the new yeast, it will just drop out of suspension, having done nothing.