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AS far as substituting your own lyrics - I don't know. I hear many songs that sound exactly like another.
There are only so many ways you can play chords - bound to be similar songs.
I wouldn't worry too much about the copyright police unless you are making big bucks off of it
Your vision is not limited by what your eye can see, but what your mind can imagine.
Make your life count, and the world will be a better place because you tried.
"Use the talents you possess, for the woods would be very silent if no birds sang except only the the best." - Henry Van Dyke