Define changing. Is it getting darker, duller, cracked, or what? If its getting darker, that's usually normal for aging. I know my spruce top went from a very creamy white to a more golden brown in just over a year now, so if thats what you are implying then it's normal.
get a hygrometer and measure the humidity in the room and in your case. acoustics do best in 45-55% relative humidity.

if your humidity is lower, then that may be the problem.
Your guitar will be fine. The wood grain isn't chaning. Those little deviations that look like the grains are stretching have always been there.
Last edited by GC Shred Off at Jan 8, 2010,