Let your Users Know
Turn off maintenance mode, and write a quick blog post to let the users know. Notice, you only spent like 15 – 20 minutes on checking things. There is no way that you could’ve catch all the bugs.Be cautious of the Sidebars
You must make sure that your new theme is widget-ready. Sidebar widgets are really easy to use therefore a lot of users use it to customize it. We notice that sidebars are probably the most user-customized area of WordPress sites. People make tons of changes such as adding custom texts, images, links, advertisement, and other widgets. If you are using a widget-enabled theme, and you switch to a theme that is not widget-ready, then you will lose all of that. If you are using a widget-enabled WordPress theme, then this is not an issue.Also anything that you modify in sidebar.php file of your old theme, will be overwritten. So make sure that you add those codes in the new theme sidebar.
Backup!!
You never have anything to lose by creating a backup. As a precautionary measure, you should backup all of your theme files, plugins, and the database. Although nothing should happen, but you can never be to safe.Maintenance Mode
You probably don’t want your users to see while you are making the switch because they will end up seeing a broken site or such. It is best to turn on Maintenance mode while changing theme. Once you have setup Maintenance mode, you are good to go ahead and activate the new theme.Testing all functionality and plugins
Once you have the new theme activated, you need to make sure that you retain all the functionality and plugins still work. Remember that good ol’ notes list that you created in step 1. This is the time where that can come handy. Go back and add any and all functionality that you want to bring from the old theme into the new theme if you haven’t done so already.Try out all the features including but not limited to the commenting process, single post pages, search, 404 page, archive page, contact page etc. Make sure all of your widgets are still there and are working.
On the plugin front, you just want to make sure that the formatting is still the same. A lot of plugins utilize your existing styles to display their output. So you probably want to make sure that they still look good with the new theme.
Make those Third Party Items look pretty
If you are using Google Adsense or another ad company that allows you to format them, then your best bet is to customize them. For example, you previous site was orange, so you had orange links for Google Adsense. Now if it is blue, then you probably want to take that into account.Same goes for twitter widget, facebook like buttons etc. Adjust those with your new color scheme. If you are going from a light to a dark design, or vice-versa, then you need to make those changes.