Run them at full blast and dump the extra energy into direct air carbon capture? Of course that would require building the CC plants but it could be planned for.
That's the whole point of creating grid-scale storage - giant batteries, lots of smaller batteries, pumped storage, pressurized air storage, traction railways, etc - or smarten up loads like EV charging so they can ramp up/down to soften the fluctuations in demand from other sources.
You'd probably just connect carbon capture plants to the grid, and run them when electricity is cheap for any reason (presumably any fossil sources have no reason to run at this time...)
On a grid with enough opportunistic loads, you'd never need to ramp down reactors.
