This is something I've been wondering about for a while.

If I have overwintered crops, such as broccoli or autumn planting
onions, at what point does the crop rotation switch over? For example,
if I sow spring cabbages now and plant them out in September/October,
should they go in the existing cabbage bed or the one that will be the
cabbage bed next year?

Also, given that the purpose of crop rotation is to avoid buildup of
disease, does it matter that although I'm moving the crops around, I'm
also digging in compost made from bits of plants from all the beds?

Rhiannon