The NET Bible gives an extremely helpful study note for this passage. Note that when it says "Yahweh" it's referring to the name translated "the LORD" and when it says "El Shadday" it's referring to what we translate "God Almighty":
There are a number of important issues that need clarification in the
interpretation of this section.
It is important to note that “I am Yahweh” is not a new revelation of a previously unknown name. It would be introduced differently if it
were. This is the identification of the covenant God as the one
calling Moses – that would be proof for the people that their God had
called him.
The title “El Shadday” is not a name, but a title. It is true that in the patriarchal accounts “El Shadday” is used six times; in Job it
is used thirty times. Many conclude that it does reflect the idea of
might or power. In some of those passages that reveal God as “El
Shadday,” the name “Yahweh” was also used. But Wellhausen and other
proponents of the earlier source critical analysis used Exod 6:3 to
say that P, the so-called priestly source, was aware that the name
“Yahweh” was not known by them, even though J, the supposed Yahwistic
source, wrote using the name as part of his theology.
The texts of Genesis show that Yahweh had appeared to the patriarchs (Gen 12:1, 17:1, 18:1, 26:2, 26:24, 26:12, 35:1, 48:3), and
that he spoke to each one of them (Gen 12:7, 15:1, 26:2, 28:13, 31:3).
The name “Yahweh” occurs 162 times in Genesis, 34 of those times on
the lips of speakers in Genesis (W. C. Kaiser, Jr., “Exodus,” EBC
2:340-41). They also made proclamation of Yahweh by name (4:26, 12:8),
and they named places with the name (22:14). These passages should not
be ignored or passed off as later interpretation.
“Yahweh” is revealed as the God of power, the sovereign God, who was true to his word and could be believed. He would do as he said
(Num 23:19; 14:35; Exod 12:25; 22:24; 24:14; 36:36; 37:14).
There is a difference between promise and fulfillment in the way revelation is apprehended. The patriarchs were individuals who
received the promises but without the fulfillment. The fulfillment
could only come after the Israelites became a nation. Now, in Egypt,
they are ready to become that promised nation. The two periods were
not distinguished by not having and by having the name, but by two
ways God revealed the significance of his name. “I am Yahweh” to the
patriarchs indicated that he was the absolute, almighty, eternal God.
The patriarchs were individuals sojourning in the land. God appeared
to them in the significance of El Shadday. That was not his name. So
Gen 17:1 says that “Yahweh appeared…and said, ‘I am El Shadday.’” See
also Gen 35:11, 48:2, 28:3.
The verb “to know” is never used to introduce a name which had never been known or experienced. The Niphal and Hiphil of the verb are
used only to describe the recognition of the overtones or significance
of the name (see Jer 16:21, Isa 52:6; Ps 83:17ff; 1 Kgs 8:41ff.
[people will know his name when prayers are answered]). For someone to
say that he knew Yahweh meant that Yahweh had been experienced or
recognized (see Exod 33:6; 1 Kgs 18:36; Jer 28:9; and Ps 76:2).
“Yahweh” is not one of God’s names – it is his only name. Other titles, like “El Shadday,” are not strictly names but means of
revealing Yahweh. All the revelations to the patriarchs could not
compare to this one, because God was now dealing with the nation. He
would make his name known to them through his deeds (see Ezek 20:5).
So now they will “know” the “name.” The verb יָדַע (yada’) means more
than “aware of, be knowledgeable about”; it means “to experience” the
reality of the revelation by that name. This harmonizes with the usage
of שֵׁם (shem), “name,” which encompasses all the attributes and
actions of God. It is not simply a reference to a title, but to the
way that God revealed himself – God gave meaning to his name through
his acts.
God is not saying that he had not revealed a name to the patriarchs
(that would have used the Hiphil of the verb). Rather, he is saying
that the patriarchs did not experience what the name Yahweh actually
meant, and they could not without seeing it fulfilled. When Moses came
to the elders, he identified his call as from Yahweh, the God of the
fathers – and they accepted him. They knew the name. But, when they
were delivered from bondage, then they fully knew by experience what
that name meant, for his promises were fulfilled. U. Cassuto
(Exodus, 79) paraphrases it this way: “I revealed Myself to Abraham,
Isaac, and Jacob in My aspect that finds expression in the name
Shaddai…I was not known to them, that is, it was not given to them to
recognize Me as One that fulfils his promises.” This generation was
about to “know” the name that their ancestors knew and used, but never
experienced with the fulfillment of the promises. This section of
Exodus confirms this interpretation, because in it God promised to
bring them out of Egypt and give them the promised land – then they
would know that he is Yahweh (6:7). This meaning should have been
evident from its repetition to the Egyptians throughout the plagues –
that they might know Yahweh (e.g., 7:5). See further R. D. Wilson,
“Yahweh [Jehovah] and Exodus 6:3,” Classical Evangelical Essays in
Old Testament Interpretation, 29-40; L. A. Herrboth, “Exodus 6:3b:
Was God Known to the Patriarchs as Jehovah?” CTM 4 (1931): 345-49;
F. C. Smith, “Observation on the Use of the Names and Titles of God in
Genesis,” EvQ 40 (1968): 103-9.
In summary, "the LORD" is God's only name and all other "names" are actually titles. The grammar in Exodus 6:3 doesn't imply that God had never told his name to the Hebrews before, but rather that they hadn't experienced the significance of the name. The Israelites were about to know God personally by seeing him fulfill his promises, whereas the patriarchs only knew him via the promises being made -- but they had in fact been told his name.
