Graham
Yes, I know when the "mass change" took place, but we are talking about the units that missed that order, or did not bother...
I cannot say a precise date when each unit finally had done the change, but since every book so far written about these codes and code changes give different views and so does basically everyone writing in on this topic, there is a clear lack of official documents left in British archives.
Again, personally, I still believe units could very well have been using both old and new codes simultaneously and especially the old codes far longer than the regulations actually stipulated, which means there is no specific date but a rather floating one
Cheers
Stig