If you're thinking about repurchasing, the real question is what you're actually paying for. Current Chanel No. 5 is still the same iconic scent DNA-wise, but reformulations happen constantly across the fragrance industry for cost and supply reasons. What you're smelling in your grandmother's bottle is partially the depth that comes with age - fragrances can shift and settle over decades, sometimes for the better. That doesn't mean the new formula is bad, just different.
The honestly harder part is the price point. Chanel No. 5 isn't cheap, and there are plenty of fragrances out there that hit similar territory without the heritage markup. If you love it enough that you wore it regularly and genuinely missed it when you didn't have it, then yeah, it's worth picking up. But if you're buying it mainly because it's "supposed to be" a classic, you might find yourself frustrated with what you're paying. The nostalgic factor definitely plays into the marketing, and Chanel knows it.
My take: keep wearing your inherited bottle until it's actually gone, and if you find yourself actively wanting more of it, grab a fresh one. You'll know pretty quick if it's something you actually need in your rotation or if your grandmother's 90s version just had something special that's tied to memory more than the fragrance itself.