I'm writing this as a foreigner. I don't know much about country music or heard that many country songs. But this is the first time I heard country music being referred to as "sad". Probably has to do with the image of classic country music.
And I don't know if I would like country music, I know I don't like country pop. But then again, I'm not a fan of pop.
Does new music change the older music? does knowing the history of music change the music?
well, that depends on how you define "different". The music obviously stays the same, but your perception of the music changes all the time.
Every time you hear a song it's different from your perspective; you have had a better or worse day than the last time you listened to it, the subject matter may appeal to you more or less with age, this time around you're really listening to that bass line.
And yes, this time you may be listening to a song after knowing more about the history of the genre or the artist or any number of things that can make your perspective different.