Difference between Patchim swallowing & consonant shift

The quoted explanation for "Patchim swallowing" is commonly called unreleased stops. You can read the wikipedia article or search for the term for more info.

I'm not exactly sure what you mean by "consonant shift", but note that there are many places where the written letter doesn't match the sound: for example, 직렬 (serial connection) sounds 징녈, because in Korean ㄱ+ㄹ always becomes ㅇ+ㄴ.

I think unreleased stops are not really a consonant shift because, in the Korean sound system, these sounds are not being transformed into a different sound: an unreleased ㄱ is still ㄱ, and a native Korean speaker will perceive the initial and final sound of 기억 is the same sound, even though the final one is unreleased and the initial one is, obviously, released.