I have an inventory pricing mod that will do this. It's meant to change prices when adding products to a category, but you can use it to do what you want to do by requesting $0 change in price. It will auto add the new category to a free category in the products or you can force it to add to a specific categoryid (categoryid, categoryid1, categoryid2, categoryid3).
I hope all of that makes sense. Might be a bit over kill for what you want, but you may find it useful too for changing your prices in the future.
Let me know if you have questions on it. Otherwise you'll have to use some mysql queries to do what you want to do.
My website link is below and
http://forum.x-cart.com/viewtopic.php?t=9823 will give you some discount coupons for this mod.
Carrie