I have checked through Internet Forums, and I found some so-called step-by-step guides for upgrading Magento CE. It seems that there are various methods each one with its pros and cons. Some people say to use Magento Connect Manager (MCM), others say to do the upgrade via command line, and others say to install the new version as a new website on the same server, transfer modules from the old to the new version and finally migrate the content and database.
I have upgraded Magento CE 1.5 to 1.7.02 on my own in the past, and it was not an easy and straightforward process, as the process was somehow successful on my test environment, but I had surprises on the production environment. I could say a lot here, but I'd just like to underline that MCM is definitely not appropriate for Magento CE upgrade, while UNIX / PHP / MySQL deep knowledge is absolutely a must.
Is there any official guide for upgrading Magento CE issued by Magento? I mean some document addressed to IT professionals including a checklist of prerequisites, step-by-step actions, acceptance testing, troubleshooting.
