migration from 2.0.16

[Barbara M.]

I have an old server with CentOS 6.x and dovecot 2.0.16 (postfix-2.6.6 and 
roundcube), that was an update from a 1.x many years ago ...
Users in /etc/passwd
Mailbox format mbox
some filtering via procmail
About 5.000 users, 1 TB data (/var/mail + /home/users)

Obviously I am searching for a smooth upgrade path (with no or minimal 
downtime and users problems) ;-)

Until few days ago my idea was a CentOS 8.x new box with the standard 
default packages (dovecot-2.3.8, postfix-3.3, ...).
Now this can be reconsidered. I suppose there will be a RockyLinux or 
something equivalent but if there is a good reason I can consider Debian 
or other OS if they have a decent EOL or some advantages.

Anyway, the more relevant problem at the moment is collect info for the 
best approch to have a smooth dovecot upgrade.

My dream is the possibility to configure a new server and rsync the data 
(/var/mail + /home/users), and, when the tests are satisfiable do the 
final sync and swap the IP, but I suppose deleting the .imap folders isn't a 
simple complete solution to compatibility problems.

Any hints, links, experiences are appreciated.

Thanks, B.