Just because Gorbag complains that the Nazgûl are "His favourites nowadays" (LR, IV, x) doesn't mean he considers them as newcomers. What is new is the importance of the Nazgûl to Lugbúrz: before the hunt for the Ring the year before, Sauron could not use them openly, and had to give orcs like Gorbag a bigger rôle. This makes sense without making unnecessarily far-reaching assumptions.
Nevertheless it is of course well possible that a few certain orcs are of the implied umairain origin, and therefore (infinitely?) longeval. However, even if Shagrat and Gorbag belonged to that category (and it is uncertain whether they do), this does not indicate anything of value for all other orcs.
|