I agree that an unopened bottle of Scotch will not change (I will concede prolonged exposure to light may effect it though, but most bottles are kept out of direct sunlight over their lifetime, and I'm sure no one has boiled or froze the bottle... lol). I am no expert; I could be wrong. But I think we can all agree it does not get better with age like a wine, which I think is what our Guest is getting at. So your 12yr is not a 35yr, it is just a 12yr. And seeing it's an inexpensive, readily available 12yr blend, I would doubt it'd fetch any real money at auction which'd be worth your effort, and worth your Grandfather's memory.
The quality shoud be good. It won't get skunky like a beer. It won't get better like a wine. It should be 'just about' the exact same as if you're Granddad had opened it 23yrs ago. That's the beauty of Scotch. You could even open it, have some, cap it, and let it sit for years, give it to your kids, and it still won't change much. However when you do eventually make a decent dent in the bottle, air will play a factor. It will oxidize. Not like as bad as a wine, but over time it'll lose it's flavours. To combat this, just put the remainder into a smaller bottle. Or just polish it off...
I do agree whole heartedly in drinking it to his honour. Get some good friends, the family you and he liked, lol, and opener then. Drinking your Granddad's bottle, telling stories about him... isn't that what Scotch is for???
If you do drink it (and IMHO I think you should) absolutely go to the shop and get a new bottle of Chivas Regal 12yr. It's inexpensive. Drink 'em side by side. It will be an entirely different drink, yet still familiar. Blends like Chivas are made to be consistent. To taste and smell the same over the years. But they can only do so much with the supplies on hand, so change is inevitable. I (and I bet everyone else that's replied thus far) wish we could taste both and tell you the differences, but we can't. So please, if you keep it and drink it, compare it, and tell us.