Here the 1874 RLDS edition changed “have mercy on this people” to “have mercy upon this people”. This reading with upon has been retained in all subsequent RLDS editions, despite the fact that the printer’s manuscript reads on. Elsewhere in the text, there are ten occurrences of “to have mercy on someone”, but there are also five occurrences of “to have mercy upon someone”. So either preposition is possible in this expression. Elsewhere the 1874 RLDS edition tended to replace upon with on (there are three examples listed under Alma 2:38), but there are no other examples where that edition replaced on with upon. The uniqueness of the 1874 change from on to upon here in Alma 19:29 suggests that this change was simply a typo.
Summary: Retain in Alma 19:29 the original preposition on in the phrase “have mercy on this people”, the reading of the earliest textual sources.