You're not in a similar position - the only person who might effectively be bothered (or amused) about you replacing that guitar with a rocket launcher, is your friend who is on the drawing. Alas, the amount of responsibility that rests on your shoulders is not comparable to that of George Lucas.
I also want to point out that I still do respect George Lucas, I just can't respect some of his actions as an artist and multi-media empire owner. Thus, I don't feel that bad to openly voice my criticism about these actions and making a little bit of fun of him from time to time - he can't expect his actions to be entirely without consequences.