Expect cleanup work. The bigger your site, the more cleanup you will have to do.
- Many of your internal links could be broken. This is probably due to the fact the Joomla 2.5 does not use sections.
- If you use a new template, you will have to reassign all your modules to the correct location.
- You might have broken images.
- You will probably have formatting issues. Many of these can be fixed with css formatting. However, if you have manually formatted content using the editor format functions, you could have a lot of cleanup.
- Because the 2.5 upgrade will change most of your URLs, you will have to create 301 redirects in the .htaccess file. If you fail to do this, all of your pages that are listed in Google will be broken links. Eventually, Google will update these URLs, but your page rank could drop if you don’t create the redirects BEFORE your new site goes live.
Bottom Line: Be prepared for some cleanup work. This is not a push-button upgrade.