No matter when the nuclear is shut off, it's going to cause an instantaneous uptick in fossil usage, because that's the dispatchable power we use. Even if you plug in an extra 4GW of solar/wind to the grid before you turn off the nuclear, the event of turning off the nuclear reactor will still make it look like fossil fuel have replaced it. (That is, until storage becomes the dispatchable replacement not only for peaker plants, but also for open cycle or combined cycle natural gas).

The event that is precipitating CA's shutdown has been planned for more than a decade. And the cost of keeping the nuclear plant would be $7B-$14B, before any of the inevitable cost increases that accompany large construction projects. $7B for solar and storage will easily replace nuclear. At $1/W for solar, and $250/kWh for storage, $7B will buy 4GW of solar and 12 GWh of storage, and cost overruns are unheard of for solar and storage installations.