I recall that the new numbers were often marked on the shopping lists/overhaul level detail, on boards adjacent to the locos, which I noted at both Doncaster and Derby Works (other works may have done the same).
As such the official date of renumbering may have been a paper exercise before the paint shop applied the vinyls? Occasionally the new number was marked on the loco too, even though it had not reached the paint shop.
So what date you consider 'renumbering' took place could have varied by a couple of months, in some cases, depending upon the level of overhaul/repair being carried out. Obviously the heavier the overhaul, the longer the period of 'floating identity'.
Some locos were renumbered by the depot and the original identity might have prevailed during the rubbing down process. Works re-numberings were always superior to those done by depots, because the loco usually had a full rub down and repaint prior to the decals being applied. Depot re-numberings sometimes were just a quick lick of blue/green paint over the old number and a new identity applied sometimes in a different bodyside location anyway. (No doubt avoided waiting for the paint to dry).
Of course many would argue that pure renumbering is out-shopping from the paint shop. Or is it?
Bear in mind that where there were serious collision damage or corrosion repairs in Works; the number panel might be missing or be new metal in undercoat in any case.