Well, I gave it a valiant effort, however despite a valiant effort, I do NOT have good news to report back on this migration. Basically after some initial Leopard setup modifications (downloading dependency modules for perl, etc), the modified mailboxes are left without cryus.index files. Ok, no biggie, just rebuild the indexes using the reconstruct tool that comes with cyrus -- unfortunately, the reconstruct -r user/david command runs through all my mailboxes and sub-mailboxes but each submailbox gets the message:

user.david.mymailfolder: Mailbox has an invalid format
....

One concern I had is that the conversion script seems to assume /etc/imapd.conf does NOT have:
unixhierarchysep: yes (which mine does).

I noticed in the reconstruct output that the few mailboxes done correctly show slashes while the failed folders do not...

user/david
user/david/Apple Mail To Do
user/david/Deleted Messages
user/david/Drafts
user.david.Sent: Mailbox has an invalid format
user.david.Junk: Mailbox has an invald format
...
notice the slashes on success and dots on failure - that seems odd...

thoughts?