Here the 1837 edition replaced the subordinate conjunction as with the coordinating conjunction and. The resulting “and he was marching forth with his numerous army” creates a semantically disconnected sentence. This 1837 change was undoubtedly a typo; as we might expect, this change was not marked by Joseph Smith in 𝓟. The 1908 RLDS edition restored the original as to the RLDS text, probably by reference to 𝓟; the LDS text restored the change in 1920, probably by reference to the 1830 edition. Similarly, the critical text will follow the earliest textual sources (the two manuscripts and the 1830 edition).
There is one place where the 1837 edition replaced an earlier and with an as. In this case, the change appears to be intended (although it too was not marked in 𝓟 by Joseph Smith):
For further discussion of this emendation, see under that passage.
Summary: Maintain the original subordinate conjunction as in Alma 51:30: “he headed Amalickiah also as he was marching forth with his numerous army”.