Here in his editing for the 1837 edition, Joseph Smith changed the singular was to the plural were. His apparent motivation was to interpret the generic whosoever as a plural so that there would be agreement with the following plural pronoun them. As discussed under 1 Nephi 17:48, whoso(ever) is sometimes treated as a singular and sometimes as a plural in the Book of Mormon text; there are also examples of mixture where whoso(ever) occurs with both singular and plural pronouns. The critical text will restore the original was here in Mosiah 25:23; for further discussion, see under subject-verb agreement in volume 3 .
This passage also shows a minor variant for the object pronoun them—namely, the reflexive themselves in the 1858 Wright edition. This form was not adopted in the subsequent RLDS text; rather, the earlier them (the reading of the 1840 edition) was restored. For the phrase “take upon them(selves)”, the Book of Mormon normally has the simple object pronoun them (15 times, including here in Mosiah 25:23) rather than the reflexive form themselves, which does occur but only twice:
It is quite possible that the 1858 error in Mosiah 25:23 was prompted by the nearby occurrence of this usage earlier in verse 12 (“they took upon themselves the name of Nephi”). The critical text will follow in each case the earliest textual sources in determining whether an object pronoun should take the reflexive form or not. Typically, the Book of Mormon text favors the older use of nonreflexive pronominal forms.
Summary: Restore in Mosiah 25:23 the original use of whosoever was, even though this singular form is associated with a following plural them; also maintain the nonreflexive them (the earliest reading) rather than the reflexive themselves.