It'll be easier to just pick a lower tension string. One of the sticky threads at the top of the STRINGS categories has a tension chart. There is also lots of info there about how different strings speak, sound and feel.
And no, changing the length of the TP wire won't effect the tension of the string but raising the action will.
Also, you want the between-the-bridge-and-tailpiece length of string to be 1/6 the playing string length to prevent wolftones. Changing the length of the TP wire would cause them. It's also and big PIA operation because you need to take all the strings off at once and the soundpost usually falls over when this happens. You also need to have some kind of clamp to secure the tailpiece wire. Whereas changing strings just releases the tension one string at a time.