Hi All,
After over a week of stuffing around in /etc/dovecot/conf.d files and lots of trial and error, I now have a controlled setup to migrate one user at a time to Maildir without affecting the other mbox based users.
For the sake of not posting an essay, if you go to this web page:
http://dmc1961.id.au/howto_17101101.php
you will see what I have done (moderators please let me know if this is the correct or allowable practice to post links rather than massive e-mails to the group?).
My sincere thanks to all who contributed and helped steer me in the right direction - I got a little lost on-line for a while but eventually got back to the right setup (our beloved Linux does have a few different file locations and setup based on the releases out there, so these were a little confusing).
Working well now on my test environment and on a test user on the client's box we need to convert, so if there are any more gotchas I will add them to the how-to web page and update you all.
Was a battle at times but it was a war worth winning.
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As always, I remain at your service.