There was indeed a doctrinal vacillation here.
In 1967, the above-cited Watchtower article declared that "Jehovah God did not grant permission for humans to try to perpetuate their lives by cannibalistically taking into their bodies human flesh," suggested that Christians who decide to avoid such "misuse of the body" are following the counsel of 1 Peter 3:16, and concluded by saying that Christians use "the divine principles recorded in the Scriptures" to make "personal decisions". This isn't an explicit command to not take transplants. Instead, the implication is that if you are a Christian, your decisions should be based on Bible principles, and since we (i.e. the faithful and descreet slave) think that Jehovah does not permit you to accept transplants, you should follow the Bible's counsel of keeping a clear conscience and thus not pursue transplantation.
But this goes way beyond a 1961 Watchtower article on organ transplants, which said that despite the fact that some religious bodies frown upon transplantation, "it does not seem that any Scriptural principle or law is involved. It therefore is something that each individual must decide for himself ... and no one else should criticize him for doing do". At that time there was no Scriptural principle against transplants, because the Society had not yet spoken out against them. But in 1967, they did precisely this thing -- declaring that Jehovah does not permit transplants. If one accepts the Society as the F&DS directed by Jehovah, there is no way a Witness would be expected to have a clean conscience if he/she goes against the Society's opinion, and now suddenly there is a Scriptural principle involved -- 1 Peter 3:16. All it took was an ex cathedra declaration by the Society to render transplants unscriptural. The 1980 article reverses this position back to the original one by declaring that "sincere Christians today may feel that the Bible does not definitely rule out medical transplants," and by affirming that "there is no biblical command pointedly forbidding" transplants -- in contrast to the earlier definitive statement that God did not permit them and the stated biblical principle in 1 Peter 3:16 that taking a transplant would have previously violated.
Finally, there is an interesting interview by Milton G. Henschel in the Detroit Free Press (July 1968) where he says directly that "Transplanting organs is really cannibalism. In transplants, you are taking something from another life to sustain your own life. ... We are confident of the resurrection and do not fear death. If a person gains another five years because of a transplant, what has been gained if he loses the future?" The clear implication here is that a person can lose a chance at everlasting life (e.g. dying at Armageddon) for accepting an organ transplant.