Here is what C.T. Russell said about "NEW LIGHT":
“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.” Zion’s Watch Tower 1881 Feb pp.3, 188
This is undoubtedly true. Something taught is either God’s truth or it was never God’s truth to begin with. Russell believed he spoke the truth. And so did the men who followed Russell after he died:
“Jehovah God commands all to worship Christ Jesus because Christ Jesus is the express image of his Father, Jehovah, and because he is the Executive Officer of Jehovah always carrying out Jehovah's purpose (Heb.:3-6)." (15 November 1939 Watchtower, page 339)
Then, in January 1954 that teaching was dropped. From that point forward “new light” contradicted and replaced what had been previously perceived as God’s truth:
“Jehovah's Witnesses say that it is unscriptural for worshipers of the living and true God to render worship to the Son of God, Jesus Christ." (1 November 1964 Watchtower, page 671)
“Reverent adoration should be expressed only to God. To render worship to anyone or anything else would be a form of idolatry, which is condemned in both the Hebrew and Greek Scriptures. Accordingly, true Christians do well to direct their worship only to Jehovah God, the Almighty." (8 April 2000 Awake! pp 26-27)
On page 13 of the 15 September 2010 Watchtower this claim was made by the men who govern the organisation: “The Governing Body publishes spiritually encouraging literature in many languages. This spiritual food is based on God’s Word. Thus, what is taught is not from men but from Jehovah."
The Governing Body must therefore believe that it was Jehovah himself who informed them that Jesus was a created being who must not, under any circumstances, be worshipped.
For what it’s worth, they stopped using the expression “new light” a while ago. They now say they are the recipients of “increased light”. This is based on how they apply Proverbs 4:18 which says:
The path of the righteous is like the first gleam of dawn, shining ever brighter till the full light of day.
Edit: Regarding the teaching that Jesus is Michael the archangel and in response to the OP question asking what new light was received and from where it came, I found this quote from the 15 September 1958 Watchtower article ‘Who is the Archangel Michael?
All relevant scriptural testimony unites to prove that Michael is none other than Jesus Christ, both before he became a man and after his ascension into heaven” (page 559).
Here is another quote, this time from 1974:
"Who, though, was the son whom God chose to be born as a perfect human creature? ...He has rightly been called Michael the Archangel. His life-force having been transferred to Mary's egg cell by Almighty God's power that overshadowed Mary meant that he, Michael, disappeared from heaven. By human birth from Mary, the Jewish virgin, he was to become a human soul." (God's Eternal Purpose now Triumphing, 1974, pp 137-138)
I then searched through the oldest Jehovah’s Witness publication I have – the 1946 book ‘Let God Be True’ – but was unable to find any mention of Michael the archangel, let alone any suggestion he was Jesus before he came to earth or after he returned to heaven. However, in Chapter 3 paragraph 4 on page 35, it says that Jesus was the Word, or Logos, of God.
From there I looked at the 1986 book ‘The Truth That Leads To Eternal Life’. In Chapter 6 under the heading ‘Prehuman Existence’ it say that before Jesus “was born on earth as a male child he served in the heavens, where he was known as “the Word,” God’s spokesman.” No mention of Michael the archangel or any suggestion that is Jesus’ name in heaven (pre or post human life).
I have spent hours searching on the official jw.org website to find any pre-1958 reference to Jesus being Michael the archangel. The problem is that the new web site does not give access to old literature. If Russell said he believed Jesus was Michael the archangel then why would he say Jesus should be worshipped? The Bible specifically says angels must not be worshipped.
So when did the Witnesses first say Jesus was Michael the archangel? Was it Rutherford who came up with this new teaching after Russell died in 1916? Does anybody know? And what “light” caused them to stop worshipping Jesus and merely give him obeisance? Perhaps we should allow Russell to have the last word as he explains why they got things wrong in the past:
“Did the Lord tell us that we would be taken [to heaven] then [1 October 1914]? No. What did He say? His Word and the fulfilments of prophecy seemed to point unmistakably that this date marked the end of the Gentile Times. We inferred from this that the Church’s ‘change’ would take place on or before that date. But God did not tell us that it would be so. He permitted us to draw that inference; and we believe that it has proven to be a necessary test upon God’s dear saints everywhere.” Source: 1 February 1916 Watchtower
I don’t know if Jehovah’s Witnesses would agree with that view, but it goes some way to explaining why changes have had to be made over the past 100 years.