The bottles or packaging might have changed over the years, but I don't believe it's been reformulated. This is one of the few fragrances where people here have not been ranting about it being reformulated.
Tester bottles very often look different than a bottle you'd buy in a store, so I think that may be the situation. The juice inside is probably the same. I've been wearing Kouros for about 16 years, and have noticed no difference in smell from the 3 or 4 bottles I've gone through.