My thought on this is simple: if a past prophet taught a doctrine and claimed it was eternal and then many years later another prophet invalidates what was taught, again, as eternal, then one of them has already led the church astray. Why? Let's say a prophet led the church for 40 years preaching xyz doctrine. During those 40 years, many people FOLLOWED the prophet, correct? Well, let's say that 125 years later, a new prophet changes everything the old prophet taught as doctrine, then all the people under the new prophet are being led astray since two prophets contradict each other in doctrine. One said the doctrine was eternal and the other invalidated such doctrine. Since God is unchangeable - at least that is what is taught - He wouldn;t teach a prophet an eternal doctrine 125 years ago and say it was eternal only to CHANGE it later and say it was temporary or a theory. You can't have contradictions in eternal doctrines or someone is not in tune and therefore may inadvertedly mislead people.