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.