Hi,
Your best bet is to go through the release notes and the Cisco bug tool.
Have a look at the open bugs for the new version and determine if there are any that may cause you pain.
Be careful however as Cisco does not necessarily make all bugs public. This is something I have noticed more recently.
There is a lot of information on the Cisco site, you really need to follow the official guide step by step to be safe.
Ensure you have a good backup before you start. And ensure you know how to recover your server from scratch if you run into a problem.
If you have servers with dual hard drives then you remove a drive before you start and use it to recover the situation if it goes bad.
Be careful of any third party applications (voice mail, contact centres, voice recording) that tie into CallManager that have dependancies and require upgrades etc.
If you are not comfortable with those procedures then I would suggest getting a certified specialist to perform the upgrade.
Planning & preparation is the key ;)
Cheers,
Tim.