When an ingredient change forces a reformulation
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A component goes out of stock, or doubles in price, or the new lot is simply different. The decision you make in that hour determines whether your blend is still your blend.
The substitution that is not one
The tempting move is a like-for-like swap: same spice, different supplier, same percentage. It is rarely like for like. Origin changes character, grind changes release rate, and moisture changes how it blends and how the batch behaves afterwards.
A substitution made quietly under time pressure is a common cause of a blend drifting, precisely because it does not feel like a reformulation.
What actually forces a change
A component genuinely unavailable rather than inconvenient. A safety or regulatory problem, such as a supplier who cannot document treatment or a material in a recalled category. A price change that makes the product unviable at its price point. Or a change in what you may claim, where a new supplier's material is treated in a way that voids an organic or non-irradiated position.
Anything else is a preference, and preferences should go through the same process as a new product.
Reformulating deliberately
New version number, dated, with the old one kept. Bench batch first, at small scale, tasted blind against a retained sample of the original. Then a full-size batch, re-qualified because mixing does not scale linearly, sampled top middle and bottom.
Only then into jars. The cost of doing this is a few days. The cost of not doing it is discovering the difference from a customer who had been buying it for two years.
Telling customers, or not
If the ingredient list changes, the label changes and the decision is made for you. If the list is unchanged but the character is different, you have a judgment to make.
The people who notice are your best customers, and they will notice. Saying that a supplier changed and here is what you did about it is a stronger position than hoping. It is also the kind of thing that only a producer small enough to know their supply chain can say at all.