@Frank Apisa,
Part 1:
The concept at the basis of the Baha'i view of changing spiritual laws is found in the Baha'i teaching in progressive revelation(PR). PR is a core teaching in the Bahá'í Faith that suggests that religious truth is revealed by God progressively and cyclically over time through a series of divine Messengers, and that the teachings are tailored to suit the needs of the time and place of their appearance.
Thus, the Bahá'í teachings recognize the divine origin of several world religions as different stages in the history of one religion, while believing that the revelation of Bahá'u'lláh is the most recent, and therefore the most relevant to modern society.
The basic concept relates closely to Bahá'í views on God's essential unity, and the nature of prophets, termed Manifestations of God. It also ties into Bahá'í views of the purpose and nature of religion, laws, belief, culture and history. Hence revelation is seen as both progressive and continuous, and therefore never ceases.
Part 2:
Bahá'ís believe that religious teachings are of two varieties: essential spiritual truth, and ephemeral social constructs. The latter may include laws of conduct, diet, institutions, ceremonies, and treatment of criminals. These may change dramatically from messenger to messenger. The former, however, are essential and do not change, except perhaps in their cultural presentation. So the Manifestation of God is seen as at once restoring the essential truth, returning the faithful to the correct practice. The Manifestation eliminates redundant or corrupt social structures and creates such social organization as will support the improvement of mankind.
The Bahá'í principle of the oneness of religion differs fundamentally from both of the traditional concepts of one of the religions in history possessing "all the truth." Bahá'u'lláh attributed the differences in some teachings of the great religions not to any human fallibility of the Founders, but rather to the different requirements of the ages in which the revelations occurred.
Moreover, Bahá'ís consider that no one of the Founders is superior to another. Shoghi Effendi has summarized this view in the following words:
The fundamental principle enunciated by Bahá'u'lláh, the followers of His Faith firmly believe, is that religious truth is not absolute but relative, that Divine Revelation is a continuous and progressive process, that all the great religions of the world are divine in origin, that their basic principles are in complete harmony, that their aims and purposes are one and the same, that their teachings are but facets of one truth, that their functions are complementary, that they differ only in the nonessential aspects of their doctrines, and that their missions represent successive stages in the spiritual evolution of human society. For more go to:
http://en.wikipedia.org/wiki/Progressive_revelation_(Bah%C3%A1'%C3%AD)