Move Yahoo e-mails from old Yahoo account to new Yahoo account

I have a friend who isn't tech savvy. She wants to move her e-mails from her old Yahoo account to her new Yahoo account. I told her that she needs to save all of her e-mails before she closes the old account.

I don't want her to install a 3rd-party e-mail backup program.

I found out online that Yahoo doesn't have a bulk export function to move e-mails from one e-mail account to another e-mail account.

So, it seems to me that her only option is to manually forward each e-mail from the old account to the new account. Is there a less time-intensive option?