In Disney canon, it has been retconned that the original Old Republic precedes the Galactic Republic, spanning over the time before the apparent destruction of the Sith, the dawning of the Rule of Two, and the establishment of the Galactic Republic as we know it. However, in A New Hope the Galactic Republic has been referred to as the Old Republic at least once.
I know a lot of retconning has happened since the original release of the original trilogy, but what is the in-universe explanation for such a change in terminology? When did this change occur in-universe?
Edit: It seems clarification is needed. My specific question is: In-universe, when did the denizens of the galaxy stop using the term "Galactic Republic", changing over to using "Old Republic" to refer to the same thing?
In the (prequel) Republic era, the Galactic & Old Republic are known to be separate entities. But during the Imperial era, the Old Republic has been redefined to refer to any Republic before the Empire, apparently. So when did this change come into force in-universe?