Spirituality : What is the difference between Gautama Siddhartha and Gautama Buddha?

Siddharth was his name before he was Initiated in BrahmGyan from his Guru Saint Sujata. After he was Initiated in BrahmGyan he saw God's divine light inside and self realized. means "उसे ज्ञान का बोध हुआ और बोध होने के कारन उसे गौतम बोध कहने लगे" but as time ped his Bodh name was changed to Buddha. like in bangali Vasu is changed to Boss etc. Buddha is Slang and in actual his name was Gautam Bodh. for example still there are many monuments which called "Bodhgaya" why it is not changed to Buddhagaya.? and Nirvana means Moksha which he attained when he left the world. and there is NOT any Tantra which he attained that was called "BrahmGyan" in which one can realized him self and see Atam and Parmatna within body. check out this site to understand what is BrahmGyan www.djjs.org