I tried to upgrade my s3 to the latest 4.1 and ran into the same problem as many others ran into: service is not available when try to manually check for the upgrades. Also can not do backup as well.
Read a lot of the posts here and in other places and seems the only way to fix it is to get a new sim card from a corporate store near by you.
And it worked on me and jut want to share with all here.
The store service is good but just quite slow. i waited there for at least 40min before a customer service dealt with me.