There were lots of LDAP & Tomcat changes made after 4.5.6, and while we test everything, just by very nature you're going to get more 4.5.10/.11 > 5.0.2 'tests'. ie: More users making that jump path because they were on 4.5.9/.10/.11 when they switched to 5.0.x
Don't know if you do, but if you have a LDAP replicas we highly advise at least 4.5.10 first, and better still is 4.5.11 see:
LDAP Replicas 4.5.x to 5.0.x - Zimbra :: Wiki
Another good article I don't see linked to elsewhere in this thread that can be used for platform changes:
/blog/archives/2007/10/moving_zcs_to_another_server.html
-Use the same ZCS version while doing the operating system change.
-Check UID/GID's.