replace is not a good word
substitute is better
so your not being replaced
but your being substituted
to be replaced would be to get rid of you all together
and every thing about you
well i can't do that
so i found a sub
someone to come in and fill your part
and remember the sub is never as good as the orignal

this was not ment to be anything but this is taken out of a letter i wrote
i just thought that this part was kind of cool so i thought id post it
It is an interesting look at the two words... I think it'd make a good concept for a poem or song, but it would need quite a bit of work. Definitely give it some flow and all that, and try using some interesting language if you do want to turn it into something. And give it a bit more length. Those would be my suggestions if you're making it into lyrics.

I like it as is too.