Any advice on big upgrade lenny->jessie?

[Boylan, Ross]

I'm recovering email from an old system that was running lenny (Debian 5) with cyrus imapd 2.2.13-14+lenny6.  I have the disks, though attached to a host running wheezy (Debian 7).  I have a jessie (Debian 8) system on which I'd like to run cyrus, with all my old mail.

The lenny configuration is
ANNOTATION skiplist
DBENGINE BerkeleyDB4.2
DUPLICATE berkeley-nosync
MBOX skiplist
PTS berkeley
QUOTA quotalegacy
SEEN skiplist
SUBS flat
TLS berkeley-nosync

any advice about how to proceed?  I am aware that only single release upgrades are supported, and that if I could get the old system running I could use imapsync.  But I'm hoping there's a better way, particularly since the cyrus version bump, 2.2 -> 2.4, isn't nearly as large as the Debian release 5->8.

1.  Would it be better to copy the files and then install the package, or install the package and then, after shutting down, copy the files?
2.  Would running in chroot be enough to get my old lenny system to do what I need, e.g., a dbdump with the old bdb?  I'm not sure running a chroot across such dissimilar system is a good idea.

Also, when I say files I'm thinking of /var/lib/cyrus and /var/spool/cyrus, but I had forgotten about /usr/lib/cyrus until getting the config info, and obviously there are files under /etc/, including /etc/init.d/, that the install scripts may use to judge that state of the system.

I'm going to take a look at the install scripts for more clues.

Originally I was just going to copy the files, install cyrus-imapd, and see if it worked.  But given the database compatibility issues that seems unlikely to work.  The mail spool is about 21G, and so I'm not eager to do stuff on it repeatedly.

Thanks for any advice.
Ross Boylan