When Oliver Cowdery copied this passage from 𝓞 into 𝓟, he accidentally misread preparatory as probationary (replacing “a preparatory state” with “a probationary state”). The source for his error was undoubtedly the probationary that occurred just before in the phrase “this probationary state”. The 1830 edition ended up with the correct “a preparatory state”, perhaps because of how the text reads later in verse 13 (“this probationary state / yea this preparatory state”). Another possibility is that the 1830 compositor originally set verse 10 as it was in 𝓟 (“it became a probationary state”), but then he later corrected the typesetting to read “it became a preparatory state” after this particular 1830 signature (the 22nd) had been proofed against 𝓞 (for the evidence of this, see the discussion below under Alma 42:31).
A few verses later, Oliver Cowdery made the same error in 𝓟 of miswriting preparatory as probationary:
As before, Oliver was prompted by an earlier occurrence of “this probationary state”, but in this case he caught his error and corrected it in 𝓟. (Initially, Oliver accidentally skipped the whole phrase “yea this preparatory state”, probably because of its visual similarity with the preceding “in this probationary state”.) In both cases (verses 10 and 13), the critical text will maintain the original reading (as found in 𝓞).
Summary: Maintain the two instances, in Alma 42:10, 13, of probationary state followed by preparatory state; in these two cases, Oliver Cowdery had some difficulty in maintaining the occurrence of preparatory in his copywork.