The Idea in Brief
The meaning here appears to be the inviolate character of the Abrahamic Covenant. That is, the Lord does not violate his own word concerning the eternal perpetuation of the Jewish peoples.
The Targum Jonathan translates this verse in Aramaic as follows. That is, there is the [Abrahamic] Covenant is the object of the Aramaic verb. The idea suggests resurrection.
According to Wikipedia, the Targum Jonathan is the official eastern, or Babylonian targum to the Nevi'im (prophets). In this regard, the Babylonian Talmud is in alignment with the Targum Jonathan.
b. Sotah 1:7, I.2, JJ-LL [Folio 8B]
II. Amemar repeated the teaching of R. Hinena bar Papa in regard to the following:
JJ. “What is the meaning of the verse, ‘For I the Lord change not, therefore you, O sons of Jacob, are not consumed’ (Mal. 3:6).
KK. “ ‘I the Lord change not’—‘I have never punished a nation and gone and done it a second time.’
LL. “ ‘And you, O sons of Jacob, are not consumed’—This is in line with the following verse of Scripture: ‘I will spend my arrows on them’ (Deut. 32:23). ‘My arrows are spent, but they are not consumed.’ ”
In other words, “they are not consumed” here appears to mean “not eradicated.”
Finally, the accent and cantillation marks of the Masoretic Text (in the Hebrew Bible) appear to place the immediate emphasis in context not on the immutability of the Lord, but the immutability of the eternal Covenant of the Lord. (Please click to enlarge the image.)
The Masoretic Text indicates that the second clause of the verse (Athnah B) modifies the first half of the verse (Athnah A), which ends with the phrase “לֹא שָׁנִיתִי,” which is simple Qal active (completed action). In other words, the remainder of the verse (Athnah B) modifies and clarifies what “לֹא שָׁנִיתִי” means, which is that unchangeableness has to do with not wiping out the Jewish peoples (and therefore excluding any possible abrogation of the Abrahamic Covenant).
The Targum Jonathan and Babylonian Talmud would appear consistent with the Masoretic reading: the sacrosanct Abrahamic Covenant in inviolate, and therefore immutable. (The nuance stemming from the Targum Jonathan would also include an allusion to resurrection in this regard.) Another inference (or assumption) would be that since the Abrahamic Covenant is immutable, the Lord would therefore have to be immutable as well (in order to ensure the complete fulfillment of the Abrahamic Covenant).
In summary, most English translations of Malachi 3:6 are acceptable, but is best understood when considered in the light of oral Jewish tradition.
Neusner, Jacob (2011). The Babylonian Talmud: A Translation and Commentary (Vol. 11a). Peabody, MA: Hendrickson Publishers, 40.