This debate has come up several times here at Delcamp. You'll have people on both sides saying it does happen, or it doesn't at all.
Personally I am a believer that guitars can wake up. After visiting various dealers and observing this phenomenon myself, I believe it is real. On the other hand, guitars sometimes can sometimes sound different for a variety of factors such as humidity, temperature, players nail condition, string quality, etc. so sometimes its hard to know the real cause of the sound difference.
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening