Here in the printer’s manuscript, we have affrightened, the past participle for the verb affrighten. The 1830 compositor set this as affrighted, the past participle for the verb affright. There are no other instances in the text of the verb affrighten, but there is one of affright:
Thus the verbs affright and affrighten each occur only once in the Book of Mormon. In the King James Bible, there are ten instances of affright but none of affrighten. According to the Oxford English Dictionary, the verbs affright and affrighten are both archaic and date from Early Modern English, with affrighten deriving from affright. Although there are citations of these verbs up into the 1800s, by that time they are archaic, dialectal, or restricted to poetry. For each of these verbs, I provide the earliest and the latest citation as listed under each of these verbs in the OED (with spelling regularized and other accidentals ignored):
affright
affrighten
As far as the Book of Mormon text is concerned, either verb is theoretically possible. The critical text will in each case follow the earliest textual sources, thus affright in Alma 52:2 and affrighten here in Alma 62:24.
The Book of Mormon text prefers the verb frighten over affright and affrighten. There are 13 occurrences of frighten in the original text. In Alma 47:29, frighten was accidentally replaced by the archaic verb fright in the 1837 edition, but the original frighten has now been restored to both the LDS and RLDS texts (see under Alma 47:29 for discussion). According to the OED, the verb frighten is a later formation and dates from the middle of the 1600s, which may explain why the King James Bible has no instances of frighten, only affright.
Summary: Restore in Alma 62:24 the archaic verb affrighten, according to the reading in 𝓟: “they were affrightened exceedingly”; this verb was replaced by the archaic verb affright in the 1830 edition, although the more common verb in the Book of Mormon text is frighten; on the other hand, affright was the verb used in the King James Bible.