Judging from other things that happened in the last 3 months, they've been planning for this for a while.
Both the audition and the whole sub-member thing both appear to be direct results of the decision to let her leave.
In hindsight, the sub-member system seems to be at least partially intended to give each of the new girls a chance to back out like Saki did.
Then since they were just sub-members anyway, it wouldn't be as big a shock if one of them decided not to join permanently.
So I guess it's not just a chance for management and Tsunku to see how the girls do, but also a chance for the girls to see if they'd be suited for the idol life.
On Oha Suta they gave Eripon the exact same costume, hair accessories, stuffed animal, and character art as Saki.
In particular, it seems like a major consideration was to avoid changing the Oha-Girl merchandise logos to match a new girl.
Instead, they made a new girl match the character on the logos by having Eripon cut her hair a while back.
Kind of odd to think that Eripon's haircut was dictated by a cartoon character design and the toy company's need to have a girl look like her.
Even with only a few days between the public announcement and her graduation, the story about Saki asking to leave in early May seems like it's true.
Too many random announcements are explained by it for it to be a complete fabrication.