Hi bukbr,
The fact that this post has been dormant for so long is probably due to the version being quite old (Posts are from 2010 and is now 2015) and most of the users have successfully upgraded.
Ignacio did respond what I believe is a good (while generalist) explanation:
There's not one single guide to solve all problems. It depends on the problem.
Here is a
guide for different types of upgrades.
If you need to perform an upgrade on an important installation and need specific help, please do PM me or contact the team.