This should help:
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 to-
day; 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)