I've heard wrist guards can make injuries worse when it comes to snowboarding.
The beauty of snowboarding is that your legs are locked, so the injury zone is mostly your upper body and arms. You will realize this very quickly your first day out, because you will be eating shit alot. For many beginners, the wrists are the biggest injury zone since your natural instinct is to throw out your arms when you fall. Don't do it, and if you do try and make a fist instead of flat-palming. I personally have never had this problem since I grew up riding motorcycles and learned how to do the body-impact roll. Try and take the impact with your body, shoulders and thighs rather then your arms.
That being said, if you find yourself being a pretty good skier, you should pick up snowboarding pretty quickly. Last year I got on a pair of skis for the first time since I was 6 and was going down black diamonds by the end of the day. One of the biggest things that transfers over for both sports is understanding how to control your edges and understanding snow conditions and what to do on different terrain. If you can use that experience to overcome the fear of committing to a turn, you will progress really fast because that's one of the biggest roadblocks to learning.
PowderHound and TreeNinja