Ironically, I just looked into the same thing two days ago. DD and DS each have a UGMA and I emailed them to ask if I can transfer it to a 529 account. They said yes, but I THINK there is a caveat. I would bet that the 529 account would need to be in my childs name, not my name. Since UGMAs are custodial accounts (you are acting in your child's behalf until they turn 18) then I doubt the money can be turned over to a 529 account in my name with my child as beneficiary. The difference in this will come when my child applies for financial aid. Money in my childs name counts more heavily (that is, they are expected to use a higher percentage of it for college) than money that is in my name. So, while this is a great alternative for the money you have in there now it might not be the 529 you want to continue to deposit into.
This is how I understand it but the finanical gurus on the board may correct me.