I have discussed Joseph Smith’s use of “Elias” to denote a forerunner, as well as his knowledge that OT Elijah and NT Elias were one and the same person in my article:
That other early LDS, personally acquainted with Joseph Smith, knew that OT Elijah and NT Elias were the same person can be seen in the following excerpt of a debate John Taylor had in 1850:
Mr. Cater has found another difficulty, which is, that in one place an angel is said to have ordained Joseph Smith, and in another that Peter, James, and John, came to him. Now Joseph had several visits and ministrations. But the difficulty with Mr. Cater seems to be, that Peter, James, or John, could not be angels. I must instruct him, however, a little, on this point also. There was a certain individual spoken of in the Bible, called Moses, he was a servant of God, a Prophet; there was also another called Elijah; they died, or were translated. When Jesus was upon the earth, he went on to a mountain with Peter, James, and John, there appeared two glorious personages, angels; Peter was enraptured, and said, "Let us make here three tabernacles; one for thee, one for Moses, and one for Elias." For who? For Moses and Elias. Here then were Moses and Elias, who had both lived on the earth, came to minister to Jesus, Peter, James, and John. Mr. Cater, I suppose, would think they had done wrong, but nevertheless they came. (Three Nights' Public Discussion Between the Revds. C. W. Cleeve, James Robertson, and Philip Cater, and John Taylor, of the Church of Jesus Christ of Latter-day Saints, at Boulogne-Sur-Mer, France. Chairman, Rev. K. Groves, M.A., Assisted by Charles Townley, LL.D., and Mr. Luddy. Also a Reply to the W. Rev. K. Groves, M.A., & Charles Townley, LL.D. [Liverpool, 1850], p. 12, emphasis added)