The first couple switch in the last (8th) chapter.
The third couple becomes reversible in psychedelics: http://www.mangago.me/read-manga/psychedelics/mi/mi-chapter-2/. They also switch in vol 2 (Souai Ero Ki).
The second couple also makes an attempt at this in psychedelics, but in the end it stops at fingering.
Where to read Souai Ero Ki: http://www.mangago.me/home/mangatopic/2074836/
Although they're not actually a couple (and idk who they will end up with).
Adapted from: http://www.mangago.me/read-manga/yarichin_bitch_club/
Ch 2-3
Ch 1-3
Read "Koi to wa Baka de Aru Koto da" (ch 5 & 5.5) first.
2nd couple
Messages
THANK U SM!!! U should check out 'Wagging the tail webtoon' they switched at the end
Haha thank you but its on the list already!!