This is almost but not quite right. Biblical Hebrew verbs don’t have tense (past, present, future) but rather aspect: perfective or imperfective, denoting actions (in this case the act of being or existing) that are or are not fully completed.

The verb in question here, אֶהְיֶה (ehyeh), is in the first-person singular imperfective, and could be translated as “I am” or “I will be,” or perhaps even better, “I am being.” The full phrase could plausibly be rendered as “I am what I am” or “I will be what I will be,” but also as “I am what I will be” or “I will be what I am,” or even as “I become what I become.” Not “I was who I was,” though; that would be in the perfective aspect.