Another good question, is what is in an older bill that is not in a newer bill? The older is probably a silver certificate, and backed by an implied reserve of the precious metal, while the newer is fiat currency, "it shall be worth $50", and has no backing.
Also, from a collectors standpoint, the old $50 is probably worth a good bit more than $50.
Doubtful. The small portrait currency wasn’t replaced *that* long ago, and as this gentleman found, can be a liability in that many people are too stupid to recognize it as legal tender.