Oh, that's a great idea. It might not even need to make the adventure necessarily objectively harder in terms of more of the same guys or higher levels, either, just more complicated with more to think about or deal with. For instance, I don't need a sidebar that says "Replace the four mook soldiers with six mook soldiers for a bigger challenge", but something like "For advanced players, there are only two mook soldiers, but a Warlock hides in the rafters, using the darkness and his Devil's Sight invocation to his advantage as he takes out the PCs one by one, starting with the one that looks like the healer."