Here in the printer’s manuscript, Oliver Cowdery initially wrote “& also a changeable being”, a perfectly acceptable reading. In fact, that expression is used a few verses later in this chapter:
Here in verse 12, Oliver’s correction of being to God was virtually immediate (there is no change in the level of ink flow for the correction). It should be noted, however, that Oliver wrote not only God in the supralinear insertion but also the connecting and (as an ampersand). Apparently, the initial reading in 𝓟 was “and also a changeable being / a respecter to persons”.
Elsewhere the text has eight other instances where the phrase “a being” refers to God:
Although most of these occurrences of “a being” are used in negative contexts, the example in Alma 26:35 is used positively. The initial use of being in Moroni 8:12 is perfectly acceptable; thus the correction to God is definitely not due to editing but is the result of Oliver trying to make his copy as accurate as possible.
Summary: Accept in Moroni 8:12 the corrected reading in 𝓟, “a changeable God”, as the original reading; Oliver Cowdery’s correction of being to God appears to have been virtually immediate; usage elsewhere in the text argues that either being or God is possible in this passage; thus the change was not the result of editing.