All truth is in harmony, whether old or new. What was true at any time in the past must be true yet.... and if it is not true now, instead of being advanced light then it was terrible darkness. [...] We have sometimes been accused by unbelievers for teaching that the true way to advance was to displace the truth we learned yesterday by new truth learned today; but we utterly repudiate the absurd charge. To grow in knowledge is to retain the truth we have and add to our stock.
(Zion’s Watch Tower July 1880 WTR p. 119)
If we were following a man undoubtedly it would be different with us; undoubtedly one human idea would contradict another and that which was light one or two or six years ago would be regarded as darkness now: But with God there is no variableness, neither shadow of turning, and so it is with truth; any knowledge or light coming from God must be like its author. A new view of truth never can contradict a former truth. "New light" never extinguishes older "light," but adds to it. If you were lighting up a building containing seven gas jets you would not extinguish one every time you lighted another, but would add one light to another and they would be in harmony and thus give increase of light: So is it with the light of truth; the true increase is by adding to, not by substituting one for another.
(Zion's Watch Tower February 1881 WTR p. 188)