Unalterable Decrees

We are reading through the Book of Mormon, with one reading from the 1840 version.  This morning, we came across a discrepancy in the text.  It was not an error being corrected.  I could not see what purpose there was in changing the text.

Alma 29:4 in the current version reads:

...I know that he allotteth unto men, yea, decreeth unto them decrees which are unalterable, according to their wills, whether they be unto salvation or unto destruction.

But, in the 1840 version, the text reads:

...I know that he allotteth unto man, according to their wills, whether they be unto salvation or unto destruction.

It is also that text in the 1841 Book of Mormon, which is the last version before Joseph died.  (https://www.josephsmithpapers.org/paper-summary/book-of-mormon-1841/332)

Why would someone add "yea, decreeth unto them decrees which are unalterable?"  I could not figure it out, until my son reminded me of the change of principles after the Prophet and his brother were murdered.  

It seems every action was calculated to boost the claim that Joseph Smith had taught polygamy, and that the Saints must accept it.  It is interesting how similar that short passage is to the words in Doctrine and Covenants 132.

Let us consider this passage for a moment:

"...yea, decreeth unto them decrees which are unalterable."

I do not recall any similar passage, that would lend another witness to this.  That God is the same today, yesterday, and forever, does not mean He does not, and cannot change His decrees.  Let me know your thoughts.