[Dovecot] 1.0.15 -> 2.x upgrade for mass hosting environment

Hi all,

We're looking at upgrading our current mail platform from 1.0.15 to
2.x to take advantage of the new director functionality.

Our main concern is the number of mailboxes involved (circa. 300K) and
the rebuilding of the index files, especially as it seems that once
you've upgraded, rolling back requires the deletion and recreation of
all indices.

Has anyone worked on an upgrade like this in the past? If so, do you
have any war stories you can share either on or off list?

Thanks in advance,

Matt