Mah Finguz!!

What will switching to lighter strings do to the sound quality?
You will tend to find you will lose some fullness. Though this may not be a problem depending on what suits you. I personally think it's good to try different guage strings just so you can experience the different sound quality.

As for callouses, the other suggestion that may help when they start developing is to use an emery board to keep them smooth. Mine tend to get a bit ratty sometimes so this helps me clean 'em up. Of course, you have to be careful not to go at them like a bull at a gate otherwise you might file away the callouses completely. ;)