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As I understand it, airlines in the USA are required to provide a 24-hour grace period after making a new reservation, during which the reservation can be cancelled or changed for free.

However, what is the rule with regards to paid changes to existing reservations? Suppose I've made a reservation a long time ago, and then I've changed it today and paid a large change fee. Do I get a new 24-hour grace period after paying the change fee, in case I've made a mistake and need to change it again? Or does the grace period apply only to brand-new reservations?

I'd appreciate an answer on both (1) what the law says and (2) what is the actual common practice with most US carriers (United is the one I'm most interested in).

The DOT legislation that requires the airline to give a 24 hour grace period does NOT apply to flight changes or change fees, only to new reservations. If you need to change your flight again within a 24 hour period of a prior change, then a new change fee will need to be paid.

Officially United Airlines does enforce a second change fee in cases like this, however if the change made was incorrect then I would suggest calling and asking for an exception - they are certainly allowed to waive the change fee. If you do not have any success, ask for a supervisor and see an if exception can be made.

The 'regulations' are on page 23129 and despite the useful context as to how they became what they are, there is nothing specific there about paid changes. However I believe the intent is clear enough and yes, there is a further 24-hour grace period (provided the reservation is still for at least one week ahead).

Accordingly, we are modifying this provision to require carriers to hold the
reservation for twenty-four hours only if a consumer makes the reservation one
week (168 hours) or more prior to a flight’s scheduled departure. After that
time, a carrier is no longer required to hold a reservation without payment for
any period of time. The Department believes that this modification strikes
the right balance between a consumer’s desire to make travel plans and shop for
a fare that meets his or her needs, and the carrier’s need for adequate time to
sell seats on its flights.

Is the revised booking a 'reservation'? IMO it is.

Is the new reservation the same as the old one? IMO it is not.

It is just as much a 'new' reservation as the example for PAX B below.

What is the intent? - a balance between consumer interests and airline economics. A revised booking by PAX A has exactly the same effect on airline economics as the same booking by PAX B (who makes it 'first time around' at the same time as PAX A's revised booking). At that time PAX B is allowed the grace period and there is nothing I can see that disallows it to PAX A.

In so far as PAX A has already made some payment (showing more sign of intent to travel than PAX B, if PAX B has not paid anything) PAX A has less adverse impact on airline economics than PAX B.

Regarding 'custom and practice' please see CONSUMERIST.