The difficult part will be removing the aging warm season vegetable plants while they are still trying to produce. That is one of the disadvantages of gardening in such an excellently mild climate. It would be easier if frost or cooling weather caused them to start deteriorating by now. Perhaps some are already getting tired. Regardless, their space is needed for new cool season vegetables.
Some of us like to amend the soil in between some of the lingering warm season vegetable plants, and add seedlings of cool season vegetable plants. Then, there is less of a rush to remove the warm season vegetables as they succumb to autumn weather. Some of us just wait for the warm season vegetables to finish, which is a delaying compromise for the new cool season vegetables.
Whatever the preferred technique is, it is now getting to be time to plug in seedlings of broccoli, cauliflower and cabbage. Kale seedlings may be added as much as a month later. Seedlings can be purchased from nurseries. Those of us who want particular varieties that are unavailable in nurseries might have sown preferred seed in flats a month or so ago, to be ready for planting now.
Beets, carrots and turnips, like all root vegetables, should be grown from seed sown directly into the garden. Roots get disfigured if grown in flats or cell packs, and then transplanted. Besides, so many individual plants are needed, that such quantities of cell packs would be expensive. Seed for turnip greens, although not grown for their roots, likewise gets sown out directly, and about now.
Seed for leafy lettuces, spinach and peas should have been sown already, but it is not too late. Kale can alternatively be grown from seed sown directly now, rather than from seedlings plugged in later. If preferred, larger heading lettuces can be grown from seedlings plugged within the next month or so. Cucumbers can be risky. If seed has not yet been sown, seedlings can still be plugged.
Whether grown from seedlings or seed, this is only the first phase of cool season vegetables. For some, later phases will prolong harvest.