When did Shakyamuni became buddha?
[unconfirmed, proceed with caution] since in his previous lives he already was a bodhisatta and so a noble disciple, becoming awakened was only a matter of time since according to the Dhamma after attainment of the sotapatti noble level (the 1st of the four) nibbana is to be attained within 7 lives at the most.

thus the Buddha was predestined to become one. even he himself couldn't change that. it's only that all his previous practice and wholesome kamma ripened in that particular life 2500 years ago



until the awakening he was not a buddha, but an unawakened bodhisatta


  before my Awakening, when I was an unawakened bodhisatta


Ariyapariyesana sutta (MN 26) by Ven Thanissaro Bhikkhu