Well you can unroot using whatever method you originally used to root or check out the multiple phones root them unroot them link in my sig. Also you will have to restore any bloatware you may have removed for the update to work.
You do not need to unroot in order to get the update. However you cannot the running a custom rom. Just flash back to your custom rom and you will be okay even if you are rooted. After you take the update you will have to root again.