As other said, houses almost never admit to reformulation, or if at all downplay it. Small reformulations can happen all the time, especially with natural materials whose characteristic can vary from year to year, and should be of no worry, if done well.
But we're talking about reformulations that change a perfume in a substantial way. Two main reasons, one is substituting more expensive materials with cheaper ones, the other one, the most important in recent years, is because of IFRA regulations (many threads about this). A big IFRA regulation came into effect in 2010, so many fragrances changed around that year (oakmoss, jasmine, heliotropin, and more).
I don't know what the specific reason for Amouage's change. I don't know whether any of the components of the original Lyric was restricted. Other Amouage perfumes have been reformulated. We know because several people have smelled vintage and current and have found them very different.