[Dovecot] Upgrading from 1.2 to 2.0

[list at airstreamcomm.net]

Greetings Dovecot mailing list! 

We are an ISP that recently migrated
from Courier to Dovecot, and are looking at the next round of upgrades.
Hoping that we could get pointed to some documentation or get a general
list of best practices for an upgrade from 1.2.13 to 2.0.9. Our current
environment consists of four Dovecot machines that share an NFS backend
hosting POP and IMAP. Our biggest question is whether we can support a
rolling upgrade from 1.2.13 to 2.0.9, or if it would need to be an all for
one cut.  

Thanks in advance.