Mediums to Lights and Back:
Proper neck relief and how to measure it

Medium gauge strings are no problem for your Santa Cruz with the caveat that the mediums extra tension may move the top and neck slightly thereby raising your action. Clockwise adjustment on the truss rod should bring it back to what you had with the lights. If you achieve the proper neck relief (see below for specs) and still find the action high this can be remedied by lowering the saddle. When changing from medium to light gauge strings you would compensate for the reduced tension by turning the truss rod counter clockwise and/or raising the saddle height with a replacement or, in a pinch, shim it up a bit it to avoid string/fret buzz. These remedies presuppose that the neck to body angle is within spec. If this is not the case it will need to be corrected by the maker or an experienced tech.

Then again things may be fine without any adjustment so skip all this and go play guitar.

Here's a trick you can try at home to determine the proper neck relief with out the supervision of an adult luthier. With the guitar strung to tension place a capo at the first fret. No capo? Enlist a passerby to press their finger at the first fret on the low E string until you're satisfied with the results. Place your index finger at the 14th fret on the low E. Halfway between these two positions the bottom of the E string should clear the top of the 7th fret by .005" to .012". If your not practiced at judging that by eye take a one, five, or ten, dollar bill, which measures four and a half thousandths, and before you mail it to me slide it between the top of the 7th fret and the bottom of the E string. If the bill gets pinched between the two your neck is too straight. If three bills or one folded in thirds slides freely through this space you are exceeding .012" and the neck has excessive forward bow. What remains in between these two extremes is the range of measurement that the player will need to achieve more or less relief depending on playing style, choice of string gauge, and height of action.

Unless you are well versed in the mechanics of your truss rod, it is best to use the above-mentioned bills to attract the attention of an experienced repair person and have them perform these adjustments for you.