The change in the 1840 edition to through is probably a typo rather than the result of Joseph Smith’s editing. As explained under Mosiah 29:1, we normally expect throughout in the context of “(the face of ) the land”. Note, in particular, the use of throughout in the very same context later on in Mormon 2:8: “and it was one complete revolution throughout all the face of the land”. The 1908 RLDS edition restored the correct throughout here in Mormon 2:8.
Summary: Maintain in Mormon 2:8 the word throughout, the reading of the earliest textual sources.