Yeri was added before red velvet's first comeback; most people believe she was always supposed to be in the group since she was featured in the happiness mv. But the happiness mv had shown the smrookies girls- Yeri; Lami; Herin and others i don't know. i think all these smrookies girls were supposed to be added to red velvet; maybe each on a comeback. irene and Seulgi were also shown as smrookies; and the smrookies boys are all part of NCT now.
Another reason is that irene was chosen as red velvet's visual at age 23. Even though she looks younger than her age; still people bring up her age often. Maybe irene was supposed to be the visual for early years then Lami would have been pushed as the visual?
Another reason is their concept Red and velvet. Red represents more energetic and youthful songs; and velvet concept songs are mature and smooth. Maybe they were supposed to become 2 subunits. Newer younger member would have been on "red" songs and older members on "velvet" songs?
Now why SM abandonded this project maybe because of debut of twice as a 9 member group which got good reception; so they decided to keep red velvet as a small group.