In any case I only have experience of dovecot and it's used in some larger installs such as the old webmail.us, now 1&1 (I believe). I think your installation is probably large enough that you might want to do a trial migration of a couple of accounts and see if migration is a problem.
I'm trying to migrate my own account from cyrus to dovecot with the 2 tools which seems to fit the most my needs :cyrus2courier : Work fast and well but I must use cyrus2courier-1.5.ts and I have 2 problems with it : falg unseen (or seen if I want) for all e-mail / Sub folders of Inbox are invisibles (I see them on the File System) !
imapsync : Must add a transition configuration to dovecot in order to have user passdb file (or master user) but once done it's ok and work correctly. I've just tested a transition and I'm happy to see it keeps all flags (seen/unseen too) and timestamp but as cyrus2courier, I can't see my inbox sub folders although I could see them (full) on the File System?
If you serve Outlook Clients and use imapsynv check that they don't see all mails with the same delivery date. There's a script on the imapsync website that fixes this problem.
p@rick
We are using thunderbird here, but I'm glad to have your advice.