Am I free to tune down and up to different tunings whenever I want? Will it mess up my guitar because of the change of pressure and stuff?
won't mess up the guitar, the strings might go out of tune more if you constantly change the tuning though.
if you do a drastic change you might have to adjust the tension springs in the back. it also depends on what kind of bridge you have too but you should be alright if you are tuning down one or two steps. but if you want to play something like korn or slipknot you might need to do some adjusting.