[Dovecot] Change mail_location for one user?

[Scott Galambos]

Hello, I'm running the latest version of dovecot on Linux with mbox 
mailboxes.  Everything works fine.  So in my dovecot config I have 
mail_location = mbox:~/mail:INBOX=/var/mail/%u

I would like to now change the mail_location for one user in an attempt 
to slowly migrate to Maildir format.  I'm confused how to do this.  I'm 
running shadow passwords:

$: doveconf -n passdb
passdb {
  driver = shadow
}

Testing any given user gives:
$: dovecot user sarah
field   value
uid     1478
gid     116
home    /home/sarah
mail    mbox:~/mail:INBOX=/var/mail/sarah
system_groups_user      sarah

What would I have to do to make only sarah's mail_location ~/Maildir 
now?  My userdb is:
$: doveconf -n userdb
userdb {
  driver = passwd
}

I tried following the wiki's but its confusing.  Thanks for any help or 
tips.