CS and SE are probably too similar and subject to the same departmental capacity limit, so it does not really make sense for them to be alternate majors of each other.
Note the change major policy at Changing Majors Within the College of Engineering - Engineering Student Services - Cal Poly, San Luis Obispo – changing into CPE, CSC from CPE, CSC, or SE appears basically unrestricted (but changing coming from other majors requires higher grades and GPA). However, SE does not allow changing into (suggesting that student look into CPE or CSC), suggesting that it may actually be the most full of the three.