Over the weekend, I finally took the plunge and did the upgrade.

It all went very smoothly
1. stop zimbra
2. took backup of /opt/zimbra
3. run 4.0.5 install
4. run 4.5.5 install

Overall everything seems to be working OK
a couple of notes
1. had to do this to get the documents working and with -l option because of certificate being out of date
2. one of the users had a very large mailbox ~18GB ,had some problems with her blackberry picking up email via pop, problem solved after reindex

One question
The spam seems to be worse than before, I was expecting the contrary, does it needs to be retrained?