Powders do change sources over time. Plants upgrade and add or drop means of production or politics stop free trade in arms-related materials with some countries or sometimes a company gets bought by an entity that doesn't want some of the original business for some reason.
Read the Accurate FAQ, items 4 and 5 (note that some automatic spell checker seems to have changed the word "from" to the word "for" in two places when referencing Czechoslovakia as a former country of origin.)
The above is one reason old load data cannot be counted on to be valid. In the case of the forced change in Nitro 100, Accurate said to stop using it in metallic cartridges at all because they hadn't done the necessary testing to compensate for the changes. They said this newer version is better optimized for 12 gauge shotgun shells, so it will burn differently. (I would have changed the name, in their shoes, but I'm not their marketing person.)
In the QuickLOAD user's manual, the author says the reason he never included data for the IMR SR powders or 700X or 800X is they have changed source plants so frequently that he does not trust any data he collects for them to remain valid.
For the most part, though, when a powder manufacturing source changes, the plant tries to keep it close enough in properties so old and new stock can share the same load data. This has happened with IMR 4198, for example, which usually comes from the Valleyfield plant in Canada. But when Valleyfield can't supply it, ADI AR 2215 has been substituted (this is according to a pre-2009 Hodgdon MSDS sheet which listed AR 2215 as a substitute for IMR 4198 and AR 2205 as a substitute for IMR 4227; that information detail is no longer supplied on the MSDS sheets).
The bottom line is you always want to run some test loads with a new lot of powder, both because burn rates and bulk vary (don't trust your old powder measure settings without rechecking) some from lot to lot, and because the source may have changed and changed some characteristic that can affect the pressure. Always check recent load data before using old load data with current production powder.