ageing or changing
This issue has been debated in about each and every whisky forum one time or another. Often one of the replies contains the impression of an 'elder whiskyman' that whiskies in closed, well sealed bottles seem to improve very slowly over time. I think that is correct.
I also think in many of the discussions on this subject opinion and fact get mixed up.
So what might happen to the juice once it is inside the bottle? At least three different things:
1) Evaporation. This causes the liquid level to recline and is a sign of an imperfect seal. This process will change the whisky, because its components will evaporate at different rates, depending on environmental conditions (water loss will be lower in more humid environments - the alcohol content may increase in very dry climates). Conceivably, the nectar might loose relatively a lot of the lighter flavor components that dissolve poorly in water. Generally, a clear sign of evaporation is bad news, as it indicates that the flavor has changed substantially because of oxidation.
2) Oxidation. This is a chemical reaction involving two compounds, one of which in our context is atmospheric oxygen (but there are many more oxidants). Because some compounds in the whisky are much more readily oxidized than others, this process can greatly affect the flavor profile of the nectar. To get an idea of this change, you might want to experiment by adding a drop of (diluted!) hydrogen peroxide, an oxidant stronger than oxygen, to a dram and see what happens. I've planned to try it but never got to do it.
3) Intrinsic changes. I believe that whisky will keep changing, even if it is kept in a completely sealed-off bottle without atmospheric oxygen and stored in the dark, as I cannot imagine the nectar is in thermodynamic equilibrium. Things change. The diamonds you have or gave to your significant other will spontaneously turn into pencil lead. Whether changes are appreciable on a time scale of decades, is another issue (don't worry about your diamonds). It might well be that these intrinsic changes are behind 'bottle-ageing'.
Many whiskies improve once the bottle has been opened for a few weeks. I have no idea whether this is because some of the undesirable flavor components (e.g. sulfur) will have evaporated or because they will have become oxidized. All whiskies will deteriorate
eventually, though. My experience is that smoky and meaty flavors keep relatively well, but that floral and fruity notes disappear quickly. Therefore, once the bottle is half empty, I pour the remainder in smaller containers (4 oz Qorpak french square bottles with polyseal caps).