A WordPress upgrade is generally pretty easy and effortless. However, sometimes things just don’t work out as planned and problems may arise. I’m going to let you know of a few problems I have found and how they may be solved.
Before Your WordPress Upgrade
First, you should always backup WordPress before upgrading. While automatic updating during your WordPress upgrade usually works flawlessly, should something go very wrong you’ll be able to restore your site. You should backup your database and your wp-content folder at least, if not all of your site.
I also recommend updating your plugins before upgrading WordPress itself. Sometimes an older plugin version may not be compatible with a new WordPress version and stop working. This shouldn’t be much of a problem, after all you DO keep your plugins and themes always up to date don’t you?
Speaking of themes, you should check that your theme is compatible with the new version of WordPress before upgrading WordPress. Some themes may not be well maintained and could break in some manner when used with a new version of WordPress. Do some research, check with the author or the folks you got the theme from. Better safe than sorry!
NOTE: When a theme is updated, WordPress removes all of the old theme files and uploads new ones. If your theme has been customized, you will lose all of those customizations! If you are unsure, before upgrading your theme, check with your web person, or if they are no longer available, check with a knowledgeable WordPress pro who can find out if your theme is safe to update.
Without further ado, here are some problems and solutions I have found when performing a WordPress upgrade to Version 3.5.
NOTE: Some of these will be (hopefully) corrected in WordPress 3.5.1.