I currently have a Fender Strat which I set up with 12-56 strings about 2 years ago and have been playing comfortably since, using mostly standard tuning. However, I've decided to switch my strings to 9-42s as the heavy set doesn't fit my playing style anymore. Because of this, I'm wondering whether i'll need to buy a new nut as my current nut has been filed for 12 gauge strings so the grooves might be too wide? I'll clearly need to do a complete setup of my guitar and adjust my truss rod, but will my nut need changing as well?
In all the guitars onto which I've put much lighter strings, (dozens, if not more) there's not been a problem. The tension will pretty well keep the strings from rattling round in the slots. Should there be a problem - very unlikely - then a tiny strip of paper will sort it out.
Obviously, the intonation will alter, needing adjustment of the saddle positions; the neck tension may need changing, and the action will probably need a tweak. You may even want to move the pups slightly higher, too.
You don't normally need to worry about the nut slots being a little wide - usually the break angle of the string over the nut plus the tension on the string is sufficient to keep the string in place. This assumes that the nut slot has been correctly filed into a suitable (roundish) U- or V- shape - if it's filed too flat at the bottom, that increases the potential for lateral movement of the string.
Try it, and don't try to deal with the problem unless you find there actually is one.