So it is with the various prophetic teachings; their externals change from age to age, but each revelation is the fulfillment of its predecessors; they are not separate or incongruous, but different stages in the life history of the One Religion, which has in turn been revealed as seed, as bud and as flower, and now enters on the stage of fruition.
Infallibility of the Prophets Baha'u'llah teaches that everyone endowed with the Station of Prophethood is given sufficient proofs of His Mission, is entitled to claim obedience from all men and has authority to abrogate, alter or add to the teachings of His predecessors.

In the Book of Iqan we read:-- How far from the grace of the All-Bountiful and from His loving providence and tender mercies it is to single out a soul from amongst all men for the guidance of His creatures, and, on one hand, to withhold from Him the full measure of His divine testimony, and, on the other, inflict severe retribution on His people for having turned away from His chosen One! Nay, the manifold bounties of the Lord of all beings have, at all times, through the Manifestations of His divine Essence, encompassed the earth and all that dwell therein.

...
And yet, is not the object of every Revelation to effect a transformation in the whole character of mankind, a transformation that shall manifest itself both outwardly and inwardly, that shall affect both its inner life and external conditions?
For if the character of mankind be not changed, the futility of God's universal Manifestations would be apparent .-- Kitab-i-Iqan, pp.

14, 240.
God is the One infallible Authority, and the Prophets are infallible because Their Message is the Message of God given to the world through Them.

That Message remains valid until it is superseded by a later Message given by the same or another Prophet.
God is the great Physician Who alone can rightly diagnose the world's sickness and prescribe the appropriate remedy.