So ...

We are going to use a handy plugin called Bulk Move. A plugin which will allow you to ...

• move posts in bulk from one category, tag or custom taxonomy to another
• disassociate posts from a certain category, tag or custom taxonomy

Here it is:

https://wordpress.org/plugins/bulk-move/

It's a very small plugin (36 KB), but let's see the usual speed tests ...

Here are my results before installing the plugin ...

... and the result after installing and activating the plugin:

No more comments needed ...

So, let's see how it works ...