Yes, you will need to recalibrate. You can either bring it in to a shop to be reprogrammed or buy a handheld tuner like the SCT XCal II and get a tune with 18" wheels.
If you went from the 235/55-17(27.2" tall:768 revs per mile)to the 235/50-18(27.1" tall:759 revs per mile)then you don't have to recalibrate.(tirerack.com info).
My SCT tuner gives you changes in incriments of 5. To me that would not be worth a tuner if you don't have/plan on getting one anytime soon.
Thanks I knew it had to be something electrical (i.e. no cable present) but I didn't know if there was a shortcut that I could use to reprogram it. Thanks for the info.
Also, good point about how close the final height of both wheel combos... I think I will leave well enough alone for the time being. No need for a tuner, yet!