The state of Buddhahood can be attained without discarding one’s present form as a common mortal by practicing the Buddhism of the Three Great Secret Laws.
Nichiren Daishonin states in the “Reply to the Wife of Lord Ota:”
[Tiantai and other great teachers conclude that] attaining Buddhahood in one’s present form is only possible through the Lotus Sutra. My disciples should never forget this. (Gosho, p. 1472)
In the pre-Lotus Sutra teachings, it is expounded that one must practice over a period of countless kalpas in order to attain Buddhahood. However, the Lotus Sutra, which teaches the principle of ichinen sanzen (three thousand realms in a single life-moment), is the supreme teaching. It enables all people in each of the ten worlds to attain Buddhahood through the power of the Law without changing their present form.
Reference: Basic Terminology of Nichiren Shoshu, p.1.