You can change the Apple ID for future store purchase by going to Settings>Store>Apple ID, tap the ID shown, sign out, sign back in using the new ID. However, all app updates are permanently tied to the Apple ID used to purchase them originally so whenever you try to update an app you will have no choice other than to enter that password or delete the app from the phone. Similarly, music and other purchased media is permanently tied to the ID used to purchase it, so the computer you are syncing with must be authorized for this ID in iTunes under Store>Authorize this computer. Otherwise the music won't play or sync to your phone.