Mixing them up would aerate them at least temporarily. It seems to me that leaves always seem to break down aerobically albeit sort of slow. While mixing them up would encourage bacteria but you'll probably be disrupting whatever fungi are there.

Why do you need to aerate them? If you're using the leaves as mulch, you won't want to disturb them. Mulches break down in time, of course, but you don't necessarily want to speed up the process.

Just FYI - "deep snows" are actually less harmful than little snow but cold temps. Snow keeps the cold wind off tender plant parts and acts as an insulator.

If you want your leaves to act as an insulator, I would add another fluffy layer. The "insulating" part comes from the pockets of trapped air between the material. Once they become compacted and start to break down, you lose most of that.

Oh, I'm sorry. I forgot the whole point was insulation.

I guess since our snows have melted - i.e. are no longer there to insulate - and we've been having winds, rains, etc. I will fluff the leaf mulch a bit and add a new layer. Whatever breaks down will, eventually, add to the soil in its own way and what's left will help insulate the area around the rootball. Can't hurt, right?

As always, thanks for the input.