Fire rated furniture is expensive. And replacing perfectly good furniture is not a pleasant or desirable option. So can they be reupholstered instead?

Theoretically yes. However this is rarely done because 1. The cost can be as much as buying a new piece when all costs are factored in from pickup to replacement of specific components and in some cases rebuilding whole pieces and 2. Often the frames used in older furniture are of unknown origin, can be particle board, or not made of the solid wood construction required to be considered fire rated.