The 1874 RLDS edition replaced unchangeable with unchanged; this appears to be a typo. The 1908 RLDS edition restored the correct unchangeable. Elsewhere in the text, we have two other references to God being unchangeable:
There is no need to make a change to unchanged in Mormon 9:19.
Summary: The text in Mormon 9:19 should read “an unchangeable Being” (the earliest reading), not “an unchanged Being” (an error in the 1874 RLDS edition).