The original manuscript is not extant here. Spacing between extant fragments suggests that to would fit best, but unto is also possible. In the printer’s manuscript, Oliver Cowdery initially wrote unto but then crossed out the un with the same level of ink flow, suggesting a virtually immediate correction in accord with the reading in 𝓞.
Most of the time, after the verb happen, the text has the preposition unto rather than to.
There are eight occurrences of “happen unto X”, but only one other of “happen to X”:
This evidence suggests that “happen to X” is a possibility, although not that frequent. Here in Alma 47:27 the critical text will accept the corrected reading in 𝓟.
Summary: Accept Oliver Cowdery’s virtually immediate correction of unto to to in 𝓟 as the original reading in Alma 47:27.