In his editing for the 1837 edition, Joseph Smith changed the should here to shall, probably because he was concerned that should might be interpreted as a conditional modal. In particular, the actual conditional use of should in the following clause (“if there should be a Christ”) probably motivated this change. Nonetheless, the change was not implemented in the 1837 edition. The 1908 RLDS edition, following Joseph’s editing in 𝓟, introduced it into the RLDS text, and this was followed by the LDS text in the 1981 edition.
Elsewhere, whenever a relative clause is used to refer to the coming of Christ, the modal verb can be either should or shall, but should is more frequent (11 to 7):
The critical text will therefore follow the earliest textual sources in determining whether the modal in any particular relative clause should be shall or should. For further discussion of the editing of should to shall, see under 2 Nephi 25:19; also see under modals in volume 3.
Summary: Restore in Jacob 7:9 the original modal should (“deniest thou the Christ which should come”), even though should can here be misinterpreted by modern speakers of English as having a conditional sense; the use of should in a nonconditional sense is common in the original text of the Book of Mormon.