[Dovecot] migration to dovecot pop3

[Rajesh M]

hi

i am planning to migrate from qmail pop3 to dovecot pop3

the number of users on the server is large over 5000

also data volume is large - over 3.5 tb -- ie many users several 1000s of
emails in their inbox

email clients used are outlook / outlook express / windows mail, etc

my questions

1) will the migration cause all the emails on the server to re-download

2) can dovecot be configured to allow only one download at a time - pop3
locking with time out setting that can be customized

3) can dovecot pop3 be configured so that it will allow download of emails
only of the last say 15 days ie even if the same pop3 user is configure on
another machine it will download only messages of the last 15 days

4) has anybody migrated from qmail pop3 to dovecot pop3 successfully
without emails being downloaded again

thankyou very much in advance

rajesh