I'm curious as well. Especially for published adventures, wouldn't it just be as simple as something like doubling the number of monsters, doubling their hit points, and doubling the amount of damage they deal (or tripling the number and damage)?
As an example encounter from Out of the Abyss, I think the PCs may encounter a handful of gnolls at around 12th level. If you know that this would be a laughably easy encounter, it takes seconds to bump up the numbers to represent a true horde of Yeenoghu, blessed by his appearance in the Underdark to all do triple damage and have triple health. Surely 40 or 50 of these buffed gnolls would provide a challenge.