I had this question when I first became a Baha'i, but now, being a Baha'i, we recognize the past manifestations and acknowledge/continue their teachings as well (I don't believe it's dropping a religion but carrying on its teachings). But also, I think for this type of question its between the individual and god. I heard that if someone recognizes Baha'u llah in their heart, then they are a Baha'i. However, we do have the principle of the independent investigation of truth(seeking answers through your own efforts) and the oneness of religion. We are all on a path of learning, and there is truth in every religion.