I finally did the upgrade and from what I can tell now, it seems to be working just fine.
I had the juggle a bit with the external authentication plugin, but opposed to the other thread, I did not have to copy anything.
Contacts and Calendars seems to be working fine, need to wait for some userfeedback, though. Fingers crossed nothing blows up now
Thanks a lot for your feedback and support!
@eredwood007: To "move over" you just need to to an upgrade/update - it works as an "drop-in".
update: Just noticed that I see duplicated user accounts in the admintstration overview, which worries me a bit.
Same username, but different fullname, group, quota, last login.
Any hints how to investigate that further?