The 1840 edition replaced the word until with till, probably unintentionally. This reading with till continues in the current RLDS text. The Book of Mormon text clearly prefers until (with at least 201 occurrences elsewhere in the original text). There is one other passage that shows variation in the textual history:
In this case, both 𝓟 and the 1830 edition are firsthand copies of 𝓞, so we cannot be sure whether 𝓞 read until or till (for further discussion, see under 4 Nephi 1:14). Finally, there are seven cases where the earliest text reads till instead of until:
So till is always possible. In general, the critical text will in each case follow the earliest textual sources in determining whether the reading should be until or till.
Summary: Maintain the occurrence of until rather than till in Alma 19:16: “even until they had all fallen to the earth” (the earliest extant reading).