When did Soetoro legally become Obama?

I legally changed my name in 1989 and have the papers to prove it, as well as a public notice published in a local newspaper. I have yet to see any proof or documentation of when president usurper, the fraud and foreigner, Obama changed his name from his adopted father's name, Soetoro, especially since I've read Soetoro was legally required to list any other names he went by when he became involved in Illinois politics. It appears Obama's deception and corruption has been festering for years.