I've a bottle of PB since forever, at least 20 years.

I suggest that you just put it in an oven or toaster oven at the very lowest heat and let it dehydrate. The physical nature may not change much, you may need to smash the heck out of it, or do mortar and pestle, but the chemical will not change.

Since PB is just a restrainer, it's not like you can really mess things up.