Mine not only changes with the weather, it changes with where I expect to be going. I walk quite a bit and it goes wherever I go.
My big packs and car pack are a different deal.

About the only thing that stays the same is my possibilities bag inside of it.
In winter my two water bottles change into one bottle and one thermos.
A spare pair of socks becomes 2 if I am traveling far. A touque gets added, a balaclava gets added. I don't use scarves, but I add a fleece sweater.
I have a hood on my jacket. Mitts get added in addition to my summer work gloves. Plus the pair of mitts in the coat pocket.
The rest of the stuff including treats is there winter and summer. The food tends to change from apples to tinned sardines in oil but that is about it.
I like trail mix a lot.
My pack is not huge but in the winter it sometimes ends up stuffed right full because if it gets warm in the day my coat goes in the pack.