After my years of probing among scores of religious movements, I stumbled across the Plain truth and the Worldwide Church of God, and became a "passive member" for some years, reading their magazines, books, brochures, and paying tithes. Later, when I met the JWs, the "leap" was not big at all, as Armstrong had paved the way - Trinity, soul, hell, birthdays, Easter, Christmas, end-time, life on Earth, evolution/creation etc. After I became a JW, I continued reading the Plain Truth for some years, and was constantly surprised to see how it covered the same topics as the WT. My guess was/is that they partly borrowed, and partly belong(ed) to the same Adventist/prophetic speculations/chronology pattern, and therefore have similar views. The latter attempt of explanation because I do not understand how he could have moles working for him inside the WT, or have the time to publish similar articles that soon after the "original" appeared in the WT.
The the Prophet died - and the Church changed all of its teachings ......... Not very deeply rooted in the hearts and minds of the followers, were they?