In this passage we have two cases where the original spelling upon should be reinterpreted as two words, up on—namely, “sent up on high” in verse 24 and “carried away up on exceeding high mountains” in verse 25. During the dictation of the text, the spelling of common words was decided by the scribe of 𝓞 (here apparently Oliver Cowdery). The correct interpretation of upon (and up on) must be determined by the context.
The 1837 change in verse 24 of upon to up on appears to be correct. Although there are no corresponding examples in the Book of Mormon itself, the phrase “up on high” is found seven times in the King James Bible:
The other Book of Mormon example (in 2 Nephi 4:25) should also be spelled as two words (“carried away up on exceeding high mountains”). Both these sentences in verses 24–25 are parallelistic and describe first an upward motion (represented by up), then a final stationary destination (represented by on).
There are at least a couple of other places in the text where an original upon should read as two words, up on:
As with the two examples in 2 Nephi 4:24–25, we have motion upwards that ends in a final stationary destination.
There is also the possibility that one or more of these four cases of upon may have originally read as up upon, as exemplified by the textual history of the following example:
For discussion of cases where up upon may be a possible emendation for upon, see Alma 2:15.
Summary: Retain in 2 Nephi 4:24 the 1837 change replacing upon with up on; emend the following three additional cases of upon (all of which involve motion upwards to a stationary destination): 2 Nephi 4:25 (“carried away up on exceeding high mountains”), Alma 1:15 (“they carried him up on the top of the hill Manti”), and Ether 3:1 (“he did carry them in his hands up on the top of the mount”).