The 1841 British edition here replaced the correct present-tense swear with the past-tense sware. The subsequent LDS text maintained sware until the 1879 edition. The 1841 typesetter was prone to make this error of replacing swear with sware, as noted under Mosiah 20:24. For further discussion of swear versus sware, see under Enos 1:14 as well as more generally under swear in volume 3.
When Alma speaks later with his son Shiblon, he uses a similar present-tense verb form in referring to this conditional promise, originally given to Nephi and Lehi (see 1 Nephi 2:20 and 2 Nephi 1:9):
In fact, Alma specifically refers to having said the same thing to Helaman (“even as I said unto Helaman”). Thus there is a direct connection between Alma 36:1 and Alma 38:1, which explains the parallel language between these two passages:
And there is no difference at all in the wording of the promise: “inasmuch as ye shall keep the commandments of God / ye shall prosper in the land”. The important point regarding Alma 36:1 is that the verb swear is a performative and is therefore in the present tense, just as the performative say in Alma 38:1 is also in the present tense.
Summary: Maintain the present-tense swear in Alma 36:1; the verb here is acting as a performative, thus the present tense; a similar use of the performative is found in Alma 38:1, where Alma directly refers to what he had earlier said to Helaman (“for I say unto you even as I said unto Helaman that …”).