Sometimes in the original text of the Book of Mormon the word retain means ‘take back’. As a consequence, some instances have been edited to regain, but not here in Alma 54:10. Moroni is determined to take back the cities and lands that the Nephites have lost to Ammoron and his armies. Note Moroni’s statement to Ammoron in verse 7 that he, Ammoron, is going to hell “except ye repent and withdraw your murderous purposes and return with your armies to your own lands”. For Moroni, the crucial goal is to regain the lost Nephite territory. So in lieu of a peaceful Lamanite retreat, Moroni is planning to attack Ammoron: “our armies shall come upon you except ye withdraw” (verse 10). As indicated by the conjunction for at the beginning of the following clause (“for we will retain our cities and our lands”), the purpose of Moroni’s military campaign is to take back the cities and lands that they, the Nephites, have lost. But since this instance of the verb retain can also be interpreted as ‘maintain’, it has never been emended. In fact, this alternative interpretation is supported by the following yea-clause, which actually uses the verb maintain, although in reference to a different sort of maintaining: “yea we will maintain our religion and the cause of our God”. Of course, that yea-clause simply makes it all that more difficult to realize that the preceding retain means ‘take back’. Yet when Oliver Cowdery copied the text here into 𝓟, he initially wrote retain as retake, which means that he correctly interpreted retain as meaning ‘take back’. Virtually immediately Oliver crossed out retake and supralinearly inserted retain, the reading in 𝓞 (there is no change in the level of ink flow for the correction in 𝓟). The critical text will maintain the use of retain here in Alma 54:10, but with the understanding that it means ‘take back’; the preceding language in Moroni’s epistle to Ammoron argues against the meaning ‘maintain’ for retain, although that meaning is not impossible.
There are seven places in the LDS text where original retain has been replaced with regain. For the list as well as discussion, see under Alma 58:3. Also see the more general discussion under Alma 44:11 regarding the various meanings of the word retain in the Book of Mormon. It is worth noting here that there is one other instance of retain that may mean ‘take back’ (but which has never been edited to regain):
But in this instance, retain could well mean ‘maintain’, especially given the language two verses later in Helaman’s letter to Moroni:
Ultimately, of course, the critical text itself does not need to decide the meaning for retain in any of these cases, edited or unedited. The original retain will be maintained in each case.
Summary: Maintain the two instances of retain in Alma 54:10 and Alma 58:10 (that is, “retain our cities and our lands”); in the first case, based on the context, the meaning is ‘take back’, although ‘maintain’ is not impossible; in the second case, either ‘take back’ or ‘maintain’ is possible.