Actually, even the Watch Tower society's "core teachings" regarding soul sleep and the trinity have been significantly altered, in that said beliefs were once considered non-essential. Behold the following Watch Tower article from 01. August 1913 (Watch Tower Reprints p. 5284):
DOCTRINES MORE OR LESS IMPORTANT
THERE are certain features of the doctrine of Christ which are fundamental and indispensable, and without which none would be recognized of the Lord as one of His followers. There are other features which would seem to be useful, helpful, blessed, but not fundamental--not essential to membership in the Body of Christ. The fundamentals have been enjoyed by good, saintlyones from the Day of Pentecost until now.
We, the same class now, have the same fundamentals, and are permitted to have other privileges, truths, "meat in due season," for our strengthening. These latter are not necessarily essential to our membership in the Body of Christ; otherwise our forefathers who did not have themwould not have been members of Christ, and there would have been no Christ Body for centuries.
The fundamental theory of the Atonement is as follows:
(1) All men--all of Adam's children--are sinners.
(2) None can be reconciled to God without a Redeemer's sacrifice.
(3) Jesus came into the world to be that Sacrifice-- and later to apply that Ransom-price for the sins of the world.
(4) On the basis of faith in the Redeemer's work, the believer may consecrate himself to the Divine service, in acceptance of the Divine invitation, "Present your bodies a living sacrifice."
(5) So doing, the believer may--up to the time of the completion of the Elect number--exercise full assurance of faith that his sacrifice will be accepted of the Father; and that he will receive a share of the anointing of the Holy Spirit--the begetting.
(6) Such as meet these conditions are to be accepted as brethren in the highest sense of the term. This much would seem to have been always necessary, and more than this we believe is not necessary today. But if by reason of our favorable day we have more knowledge, we may also have corresponding trials, which our greater knowledge will offset.
Our advice to the Lord's dear people everywhere is that they put no yoke upon each other, beyond the fundamentals specified above--that otherwise they stand free, and leave each other free, and fellowship and agree as much as they can with each other.
