I've noticed whisky does change once the bottle is opened. Seems like it doesn't take long at all... a few days even.
I originally signed up here to post my experience with storing whisky in flasks, since I couldn't find a post anywhere online of anyone keeping whisky in a flask more than a few days.
Last october I filled a flask with 10yr Laphroaig and it sat until now (mar 19th). That's 5 months. I had a sip and noticed a plastic/musty like taste. This is 86 proof stuff... shouldn't be anything growing in it. So I searched online for opinions about flasks, whisky storage, mold, etc and ended up here. I was about to post when I decided to get the original bottle and have a few sips to make sure the flask is really at fault. Well, the original bottle tastes just like the stuff in the flask! I can't taste any difference whatsoever. They both taste like crap. Well... crap with a sweet aftertaste. Anyway, I was all set to blame the little plastic foamy thing in the flask's cap that provides the seal, but now the only thing I can think happened is the whisky just expired.
The funny thing is... the flask was full. Almost no air at all inside. The flask is stainless, so no light got in. It was stored in a cupboard at house temps during winter right beside the bottle which was about empty with plenty of air inside. Some things just don't last... even if they are bottled properly. Islays have lots of complex chemicals that could possibly change on their own without air or light.
I know pepsi tastes terrible if the expiration date isn't at least a month away. Its March now, so if the bottle doesn't say May or at least late April, I won't buy it. Coke isn't like that, just pepsi.
I figure some things just naturally evolve, breakdown, oxidize, and react with other things which results in a change in flavor. Islays are very rich in flavor, so I figure they're more prone to flavor variances than the chill-filtered highlands.