What was the reason that Ku Hye Sun almost missed out on starring in the popular TV series ”Boys Over Flowers”?

Ku Hye Sun was about to lose her role as Geum Jan Di. On the 24th, KBS2TV's "Park Won Sook's Let's Live Together" aired.
Season 3' depicted conversations between Park Won-sook, Hae Eun-hee, Ku Hye-sun, and Kim Miryo.
On this day's broadcast, Ku Hye Sun spoke about her experience as a singer in the popular TV series "Boys Over Flowers" broadcast in 2009.
Ku Hye Sun surprised everyone by saying that she almost couldn't become Geum Jan Di.
Ku Hye Sun said, "At the audition venue, we were saying to each other, 'You're too old.'"
They were underage, and I was 24 at the time," she said. "So they said, 'If you don't like me, you don't have to have me in the show,' and I said, 'I'll go first, and I'll get back to you later.
I said, "I'm going to go home first, but the scriptwriter stopped me and said, 'This is the character,' and I was asked to leave the role.
"The project was carried out in a timely manner," he said.