Can old cigars be rehydrated?
Yes.
I just did this with a couple of old boxes that my brother in law had. I used some of the higher percentage Boveda packs...like the ones you use to season a humidor...and sealed them up with the cigars in a mason jar. Left it for a couple of months, opening it long enough just to check the packs and get some new air in every couple of weeks. Worked like a charm.