How much I once suffered with upgrading Joomla… I had to update several sites from Joomla 1.0.x to 1.5, then to 2.5. Often it was all done semi-automatically – when the sites were too big. It was the quiet horror – was going encoding, came out a wild array of errors in the database. Now, thank gods, everything is easier – there are specialized components, and Joomla itself to update itself learned. Just recently updated Joomla from 2.5 to 3.x. In default mode, not everything worked, in the end used a natural free component redCOMPONENT. In general, without a hitch without a hitch.
The main problems when upgrading Joomla:
1) they don’t do backups. No comment.
He who did not make a backup – he made a f*ckeup
3) The third point follows from the second. Do not update the site to live, make a new database, a test subdomain and do your dirty work there.