Most modern perfumes are different from the ones that originally bore the name. Seasonal fluctuations in quality and pricing of natural products can create slight variations but the largest sources of change are the development of cheaper synthetics and the banning of ingredients by both government and industry-made regulations. It's a great hazard to perfume lovers that perfumes can be changed at any time for any reason with no notification from the manufacturer. Almost every iconic perfume has been changed to take advantage of modern synthetics, sometimes with no real dip in quality (Chanel No.5 is still relatively untouched I think) and more often marred into a shadow of its former self (Cabochard, Bandit, Tabac Blond etc etc ad nauseum).
Even if you like the current version of something it's worth getting your paws on a vintage version; there may be little difference between the two or they may be unrecognizable. For whatever reason I find perfumes' original incarnations are usually better than the later ones and I've yet to smell anything that's been reformulated from awful to excellent.