Five minutes here saves an evening of undoing things later. What you need to do depends on which of two situations you are in.
#Which situation are you in
- A new, empty site
Run through the basic checks further down and move on. There is nothing to lose, so there is nothing to protect.
- A site that already has content
Work through the whole page. You are about to change the theme on a live site, and the current theme takes its settings with it when it goes.
#On a site with content
#Back it up first
A complete copy of the files and the database, taken before you touch anything. Not an export of posts: a full backup, stored somewhere other than the same server.
This is the one step with no substitute. Everything else on this page is about avoiding trouble; this is what gets you out of it.
#Disable third party plugins for the installation
This looks like unnecessary work and it is the step that saves the most time.
With plugins off, you can see what the theme does on its own. Anything that then goes wrong belongs to the theme, and anything that only appears when a plugin comes back belongs to that plugin. It also removes the most common source of conflicts during a demo import.
Deactivating a plugin does not delete it or its data.
Everything comes back when you activate it again, so turn them back on one at a time after the install and look at the site between each one.
#Write down your current setup
The old theme takes its own settings with it, and reconstructing them from memory takes longer than writing them down:
- which menu is assigned to which location
- which page is set as the front page, and which as the posts page
- what is in each sidebar or widget area
- any homepage layout choices you would want to reproduce
#Freeze your permalinks
If your site uses a non default URL format, note it now and do not change it while switching themes. Changing the permalink structure on a live site breaks every existing link to your content and the search rankings attached to them.
#Consider a staging copy
The calmest way to do this is to install the theme on a copy of the site first. If your host offers staging with one click, it is the best five minutes you will spend.
One thing to know: a staging copy is a separate domain, and every licence carries a free slot for exactly that.
#Basic checks for any site
Permalinks are set to a readable format, not the numeric default
Search engine visibility is not accidentally switched off in SettingsReading
You have file access to the site over SFTP or a file manager, in case the admin install fails
Time zone and date format are set, because they show on every post
The server meets the requirements
Caching is off, or you know how to clear it, for the duration of the install
That third one matters more than it looks. When a theme install goes wrong, the recovery is almost always through file access, and discovering you do not have it in the middle of the problem is a bad moment to find out.
#What not to do on the same day
Change one thing at a time, or you cannot tell which one broke the site.
Do not move the site to a new domain while you change the theme, do not install ten new plugins the same afternoon, and do not start on a Friday evening, which is less of a joke than it sounds: the fix usually needs somebody at your host, and nobody is there until Monday.
#Common questions
Will switching themes delete my posts?
No, because posts, pages, media and users belong to WordPress rather than to the theme, and what you lose is theme specific settings, widget placements and menu assignments.
Can I preview the new theme without activating it?
Yes, the Live Preview button in AppearanceThemes shows the theme against your real content without switching visitors over, and while it will not show everything a full setup does it answers the basic question.
How long does the switch take?
The installation itself takes a couple of minutes, and setting the site up the way you want it is the part that takes real time, which a demo import shortens considerably.
Something is already broken before I start
Fix it first, because installing a theme onto a site with an existing error makes that error look like the theme's fault and costs you the one clean comparison you had.
#Still stuck
Open a support ticket before you switch if you are unsure about any of this. It is a much shorter conversation than the one afterwards.
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