[Dovecot] Migration

Hi,

I have a Mailserver setup with Qmail+Vpopmail+courier-imap with 2000 users.The authentication in vpopmail was in plaint text file based.Right now the issue is while opening a user email box with 50-100 mails it is opening fastly.Butfor email-boxes with attachments and increased number of mails it take long time to open the Imap connection. So I would like to change the Imaps to Dovecot.Please suggest if I can migrate from courier-imap to dovecot with vpopmail and qmail without changing the present username and passwords.

Thanks in advance for any help.

Regards John