I think if manufacturing process changes significantly (lord help laywers define "significantly") it should be considered a fraud to not notify customers in clear and explicit manner.
if it just says "shatterproof" or "break resistant", then that is rather vague of course, but that should be a warning in itself. (of course, brands are a thing, we are lazy after all, an re-evaluating products every time seems like a waste of time)
I say that it should be "10 (but was 20 a year ago)" and no fine print allowed for second part.