Here the 1874 RLDS edition replaced the preposition upon with the more modern on. The 1908 RLDS edition restored the original upon. Notice that the immediately surrounding text uses the preposition upon in the same phraseology:
Elsewhere in the text, there are 17 more instances that refer to wrath being “upon someone”, but none of wrath being “on someone”. For this expression, the Book of Mormon text consistently supports the use of the preposition upon. (For other instances where upon and on have been mixed up in the history of the text, see under 1 Nephi 12:4.)
Summary: Maintain the original preposition upon in Ether 2:9 (“and the fullness of wrath cometh upon them”); elsewhere the text consistently uses upon, not on, for this expression.