Here's a checklist of items that can help things go smoothly when upgrading to cPanel 11:
1) Make sure you're running Perl 5.8.8:
a) check perl -v to make sure it says 5.8.8
b) if you see 5.8.7 or something else, follow these steps:
1) wget
http://layer1.cpanel.net/perl588installer.tar.gz
2) tar xfvz perl588installer.tar.gz
3) cd perl588installer
4) ./install
5) /usr/local/cpanel/bin/checkperlmodules
2) Upgrade YAML::Syck:
1) /scripts/realperlinstaller YAML::Syck
3) Remove any SARE/openprotect spamassassin rules
These seem to be not compatible with SpamAssassin v3.2 see
This Thread for more info.
4) Upgrade from mbox to maildir.
This can be done by running /scripts/convert2maildir.
WARNING: Make sure to run the option to back up your mail first.
When running the conversion, you should watch out for the following:
**IT WILL TAKE A LONG TIME. Don't kill the process or you'll be left with a lot of half-converted mailboxes and angry clients who aren't getting mail.
**Mail will not be delivered during the conversion, it will be instead queued until the conversion is complete and then delivered.
**Make sure your mailbox permissions are ok before converting and make sure your clients aren't currently using mailboxes called /cur /new or /tmp
These 3 steps should make things a lot smoother if you are running an older version of cPanel.