If fermentation is complete, then colder temps during secondary fermentation are actually preferred (even for ales). It'll help precipitate more matter out of the beer, granted, not a major consideration for a stout. But, it'll age better.
The only issue is if you happened to have racked before fermentation is complete, the yeast may go dormant. Did you take a gravity reading when you racked?
But if it was done fermenting - GREAT!