Hi @nobuhle,
Thanks for the great question!
I checked out some commentaries and both Bovon in his Commentary on The Gospel of Luke 1:1 - 9:50 in the Hermeneia series and Stein in his Luke commentary in the NAC series, focused on the literary/symbolic meaning of these men representing the fulfillment of the law and the prophets (Luke 1:1 - 9:50, p. 376; Luke, p. 284) with Stein also noting that, “The presence of Moses and Elijah refutes the incorrect guesses about Jesus’ identity given in Luke 9:8, 19,” (Luke, p. 284).
Neither, however, address this question concerning the mechanics of their appearance nor the transfiguration’s impact on the theology of the dead.
From my previous studies concerning the state of the dead (I haven’t yet been able to look into the transfiguration specifically), my tentative explanation would be that Moses was taken from Sheol, which is not beyond God’s reach as demonstrated by the resurrection and attested by Psalm 139:8, and appeared in a - I would lean toward - disembodied state, perhaps like that of the angels, at the transfiguration.
I say disembodied, because even though he appeared as a man, it would seem strange to me, given 1 Cor 15:20 and 1 Thess 4:13-18, that he could have received a glorified body before Christ. Moreover, that a disembodied spirit (if Moses was such) could be mistaken for an embodied being is attested to in Luke 24:37-39 and Acts 12:15.
I do believe though that it is possible to interpret 1 Cor 15, 1 Thess 4:13 - 18, and 2 Cor 5: 1-10 in such a way that would not conflict with Moses appearing in an embodied state. Particularly, many arguments are made concerning the present tense “have” in 2 Cor 5:1-10 to argue for immediate resurrection or an intermediate body. (Check out Murray J. Harris’ commentary on 2 Corinthians for an excellent discussion of the Greek text and the various possible interpretations).
Overall, however, I am not convinced by such arguments and thus prefer a disembodied interpretation of Moses’ appearance.
As for Elijah, he never died, but was taken up to heaven in a fiery chariot 2 Kings 2:12. As little as the Bible says about the state of the dead before the return of Christ, it says even less concerning the state of Enoch and Elijah. So, whether Elijah lives in his earthly body, a disembodied state, or something else I think is anyone’s best guess.
Though he doesn’t say anything that I could find concerning the transfiguration, if you’re interested in further study I thought Paul Williamson’s book Death and the Afterlife: Biblical perspectives on ultimate questions was an excellent monograph.