I went back to Yatagan too, after several years. The longevity seems less on me also but I don't think the formula has changed. The last time I used it, it was splash and I'm sure I was simply wearing more. Also, my nose is much more educated about picking apart the notes so I'm noticing new things. That's what I really think it is.
Caron has always re-packeaged their scents--and often--and I don't think they reformulate them every time.
BTW, if you like Yatagan, try (if you haven't) Aramis Devin. Very similar but softer, gentler, kinder.
Every once in a while, when I happen to wear either one at the right moment, with the right mood, weather, temp, humidity...they're magic. Of course, this applies to any scent as far as I'm concerned.