Forgive me for this, but it's the first time i've snapped a string.

At the moment i'm on D'Addario 10s, and my D-string just went out of the blue (don't ask how). I'd like to switch to 9s for easier bends but will this require me to setup my guitar again?

Also, string recommendations/crash course in "bright" and "nickel wound" etc?
You may need to rejig the action for a lower gauge. I can't you'd need to adjust relief as there will be less tension on the neck, so yeah, action I reckon.

Strings is all personal preference. I had some D'Arddario bright strings on by guitar before and I couldn't wait to get them off, as I prefer a darker sound.
Thanks, i think i'll take the "if it ain't broke, don't fix it" approach and stick with 10s.