
Although the migration process is quite safe, I'll still advise you to take a back up before attempting a switch. You can either take backup with the help of a plugin or can go for a command-line solution.
As always, I'll recommend once testing the migration process on a development site instead of directly going for the live version. Without wasting any time, let's learn how to make a switch to ClassicPress.
Things to Know Before You Start
ClassicPress is a fork of WordPress 4.9. Therefore, all the WordPress plugins and themes compatible with that version work seamlessly with ClassicPress too.This doesn't mean that plugins and themes released for a newer major and minor version of WordPress don't work with ClassicPress. Most of them work, out-of-the-box.
There's also a plugin directory for ClassicPress in case one of your plugins isn't playing nice with your migrated instance.
Back-Up and Migration Steps
Before you start the migration, take a full site backup to ensure you can roll back in case of any mishap though chances of the same are almost negligible.Thereafter, download the ClassicPress migration plugin from their site.
Within your WordPress dashboard go to Plugins → Add New option and click the Upload Plugin button.

Select the ClassicPress migration plugin downloaded earlier and click the Install Now button.

Finally, click the Activate Plugin button to enable the ClassicPress migration plugin.

The plugin is now installed and activated. Now, click the Switch button as shown in the image below.

Here, I'll suggest you do this migration at off-peak hours during the weekends or whenever the traffic on your website is at its lowest. It'll ensure the visitors don't see a momentarily garbled site while the migration is going on.

Now, you'll get a splash screen with some messages. The plugin performs some critical checkups before moving ahead. You may see some warnings on this screen.
If you've edited the majority of your content with the block editor, then you may need to review and find a strategy to deal with the content formatting issues which may arise after the switch.
Now click the big blue button shown in the image above to kickstart the migration process. Next, you'll be presented with the database upgrade screen.

Simply click the Update ClassicPress Database button. The next screen prompts you to finish the update process by clicking the Update button.

And, that completes the entire migration process. Make sure to immediately visit your website to see if everything appears right on the front-end as it should be.
In case a plugin is not working, visit the ClassicPress plugin directory to find an alternative of the same.