Mouse, I think what I'd be sure to caution against is blind changes.

Using your example, if I were to take the chorus, /I-IV/I-V/I-IV/I-V/I-IV/I-V/IV-I/IV-V/

and keep the vocal line, but change the Chords to:

/I-vii/VI-V/I-vii/VI-V/I-vii/VI-V/vii-I/vii-I

I would still be vaguely "right", in regards to the tonic, if I assumed that the Vocal line was written from the tonic, but as you added on the incidental "passing" notes that the lyrics and tune dictate, it would sound miserable.

To Marion, I think I would just say, a little theory goes a long way, there is a very simple fixed formula that will help you out, BEADGCF (the cycle of fifths), and a primer into Tonic, Dominants, Sub-Dominants, and Secondary Dominants are all you need to do damn near anything, and yes, above all, do what sounds good... :)