Why does it take so long?
Once it’s cracked, the pot has to be completely cleaned out and relined which takes weeks. A smelter usually has hundreds of pots so this alone takes a while as the liner and anything in it are basically frozen solid and need to be broken apart and torn out. Once relined the pots must be brought back up slowly and the chemistry balanced. The pots also draw a ton of power and are wired in series so they have to all be brought up slowly together (or in batches).
That assumes it was a clean shutdown with nothing else clogged up in the system. “Cleaning” in smelting means that the hardware involved needs to be replaced because it fused to molten metal while cooling down.
You also can’t fully drain a pot. You can siphon most of the aluminum and cryolite off but at those temperatures they behave like a proper liquid with surface tension and the metal wicks into the pot like solder instead of flowing with gravity.
To keep it running at reduced capacity will likely be less expensive unless the war goes on for a very long time.
The smelting process I described above is actually the more expensive process to used to produce aluminum from raw bauxite. Recycling aluminum is cheaper and a significant fraction of the world’s aluminum produced every year is from recycled feedstock (over two thirds in the US, last I checked). Same goes for steel and most other metals.
if you make it cold, you'll have to do whole startup sequence again