I assume the OP is based in the US?

Allowing standby travel for an earlier flight seems to be normal practice in the US. In Europe (ie on European airlines) it is not, and it depends on the fare class. If your ticket is non-exchangeable, then you can't exchange it, even at the airport, and even if the flight before yours is empty. (The argument is that otherwise, no-one would pay the extra for a flexible ticket.)

In practice, you might be lucky if the flight on which you are booked is full/overbooked, and the earlier flight has plenty of space, which means that by moving you, they are clearly benefiting someone waitlisted on the flight you are booked on.

But otherwise, they are effectively giving you a benefit for which you haven't paid. It would be like giving discretionary lounge access if your ticket doesn't entitle you to it.