Well since those 2 are explicitly in the poster I definitely would expect them to stay for quite some time at least. Posters doesn't usually show human characters having too minor of a role IIRC.
As already pointed out:
Being on a poster doesn't necessarily mean you play a major role. Acerola and Kuchinashi were on the previous poster despite playing a much smaller role overall than Lychee did, and Hikari was on the Best Wishes Season 2 poster despite leaving permanently in episode 10. Ibuki was on the Decolora Adventure poster despite appearing in ONE episode.
Being on a poster doesn't really mean more than the marketers wanting to call attention to the fact that you're going to appear.