I'm not saying make the spawns tougher. They would retain their current CR, only have fewer creatures. I'll use the spider spawn in the Sielwood as an example. Gilli [level 10] normally spawns 2 small spiders, 1 or 2 giant spiders, a phase, and dire spider on top of the ettercap. So thats a total of 6 or 7 creatures. I don't know what the appropriate replacements would be, but just to explain the idea I had, the original spawn would change from 6 or 7 varied spiders, to maybe 2 or 3 Dire spiders and the ettercap.