I have never heard of the factory, or have seen a factory parts bulletin (and I have a pretty complete set of them), making available an update kit as you describe. As a dealer, we had been modifying the stock 1966 and 1967 front brake, using modified brake cams, to make it a twin leading shoe. It is fairly simple thing to do.
When the 1968 twin leading shoe plate became available from the parts department we were buying them and offering them as an accessory for earlier models.
Very few people at that time were buying a bike and thinking, how can I preserve it so someone 50 years from now can see what it really looked like. People love to personalize THEIR motorcycle and all sorts of changes were made to do accomplish this. We even sell retro fenders, headlamp ears, etc. to make T140s look like pre-1970 models. Will someone be saying 50 years from now, "Look, there is a T140 with a factory T120 kit!"