When can changing a theme damage a website?

My supervisor thinks that changing the theme of an existing website is as easy as clicking a button.

To my way of thinking, depending on the change of theme, areas for posts or pages (or other things) might be added, eliminated, or the layout may stay the same (only the colors or background images may be different, for example). In the case that things get eliminated, if that can happen, is the WordPress developer alerted at all that something bad may be happening, or could the changes eliminate existing work?

In the case the changes eliminate existing work, how would one be able to know (besides scrutinizing the entire web site) how things would be affected. Is it possible to do this before hand?

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In short yes. The best way to achieve this is by setting up a staging environment. This will have a carbon-copy of the latest version of your WordPress site. From there, you can update the theme and verify that everything works as planned.

Once everything checks out, you can apply those changes to your live environment. See this article for more details.

This is about trust level in the developer.

Full paranoia mode – you don’t trust at all and you not only read the change log but also inspect the code. This is the only way to go when any down time means a noticeable loss of money.

Full trust – the themes are from a reputable source you trust, and they claim that things are compatible enough to change the themes. In this case you just go and change the theme. (note, reputable here means they understand user expectations, and have a proper QA, not that they never have bugs)

The (very) wide in between – it is all about risk calculation, how much money/time will you loss if you do the upgrade and it will fail so badly you will have to restore the site from backup. For some sites the cost of setting proper testing environment might be higher.

But as you imply the kind of content you have on the site have a huge impact on the ability to switch themes. In general if you want to be able to change themes easily you should avoid using any of a theme’s specific widgets, shortcodes, post meta data, and CPTs. Themes should be used only for front end design, any content related settings should be done in plugins.
If you follow this path when setting up a site, a themes switch should be almost painless with the main risk is incompatible sidebar and menu area names which might require some small effort to re-attach things to where they should be.

To change the theme name you need to changed it in the style.css file also the folder name but that’s not it if you have given static path somewhere in the files it can cause issue. What you can do is just copy paste a theme and change it’s name and try to activate that new theme if everything goes well you can take that backup of older one and remove it. hope it’s help.