In these two verses, the text has varied between be aware and beware. In verse 17, both 𝓟 and the 1830 edition read be aware, which was emended to beware in the 1874 RLDS edition. 𝓞, not extant here, probably read be aware since for this part of the text 𝓟 and the 1830 edition are firsthand copies of 𝓞. The RLDS text has maintained the secondary beware, while the LDS text has maintained the earlier be aware. In verse 18, on the other hand, we get variation in the two earliest sources, be aware in 𝓟 but beware in the 1830 edition. The 1830 reading has been maintained in both the LDS and RLDS texts. Thus in the current RLDS text, both instances read beware; in the current LDS text, we have a mixture, first be aware, then beware.
In both verses, the meaning seems to be beware rather than be aware —that is, the context implies the stronger ‘to be on guard’ rather than the weaker ‘to be cognizant’. In addition, usage elsewhere in the Book of Mormon supports beware when followed by a lest-clause (although there is only one example):
This usage is also supported in the King James Bible:
In contrast to this usage, there are two instances of “to be aware” in the Book of Mormon; each has the meaning ‘to be cognizant’:
(The King James Bible itself has five instances of “to be aware” with the meaning ‘to be cognizant’.)
All of this evidence argues that the expected reading for both instances in Mormon 8:17–18 is the stronger beware, with the result that the use of be aware (both times in 𝓟 and once in the 1830 edition) seems inappropriate. But this makes one wonder why the earliest text favors be aware. The difficulty of be aware seems to suggest that be aware is textually correct and that the tendency has been to replace it with the expected beware (as in the 1874 RLDS edition for verse 17 and in the 1830 edition for verse 18).
In Early Modern English, the phrase “to be aware” actually had the stronger meaning ‘to be on guard’; in other words, it was basically equivalent to “to beware”. Under the predicate adjective aware, the Oxford English Dictionary lists the first (and original) meaning for aware as ‘watchful, vigilant, cautious, on one’s guard’, with these two examples from Early Modern English (accidentals regularized):
The OED identifies this first meaning as obsolete (although examples of its usage extend into the first half of the 1800s). Under definition 2 for aware, the OED lists the modern meaning, the one that English speakers expect for “to be aware”, namely, ‘informed, cognizant, conscious, sensible’.
What we seem to have here in Mormon 8:17–18 are two archaic uses of “to be aware” with the meaning ‘to be on guard’ (that is, the same meaning as “to beware”). It would appear that in verse 18 the 1830 typesetter unintentionally made the change (from the unexpected be aware to the expected beware) since the change was not made to the first instance of be aware (in verse 17). Later, the 1874 RLDS edition made the change to beware in verse 17, perhaps because verse 18 read beware. The critical text will adopt the earlier be aware for both cases, but with the understanding that it means ‘beware’.
Summary: Maintain in Mormon 8:17 the earliest extant reading, be aware (the reading in both 𝓟 and the 1830 edition); restore in Mormon 8:18 the reading in 𝓟, be aware, in place of the 1830 reading, beware; in both verses, be aware has the obsolete meaning ‘to be on guard’, the same as beware.