Blends do change. In fact ALL blends change over time, and for a lot of different reasons.
1) Blends change because the ingredients change. Tobacco leaf is a natural product, so it is never the same, season to season. Sometimes, a given leaf will go out of production, or just become difficult to obtain in sufficient quantities, so substitutes are made.
2) Blends change because the processes change. Lots of different reasons for that, ranging from altering production to meet increased/decreased demand, government labor and safety law dictating equipment changes, to bean counters seeking greater efficiencies.
3) Blends change because the customer's tastes change. Producers will change a blend when sales drop and focus groups suggest changes.
4) Blends change because manufacturers change. Almost all of the blends/brands that we smoke (with few exceptions) have been sold within the past 15 years. Production gets moved, procurement changes, everything can change except for the brand name.
There are probably a bunch of other reasons, too.