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https://christinacreativedesign.com/how-to-bulk-replace-the-4-8-wordpress-text-widget-with-the-classic-text-widget/

Here is a much faster and safer way to bulk transfer if you are not adept at using phpMyAdmin and tweaking wp_options table (and possibly others) for serialized data (which is tricky to work with).

First get the new plugin:

Download the Classic Text Widget Plugin 

Install it and activate it using the upload plugin button. Read your screen.

Then install and activate the Widget Importer Exporter Plugin from the WordPress repo.

Using that plugin, export your current widgets to a .wie file.

On your local drive, duplicate the .wie file that was generated and rename it something else. Open that renamed .wie file in a code editor (not a word processor).

Find and replace all using the following 2 strings (exact). Make sure you are at the top of the file.

Find:

"text-

Replace:

"classictextwidget-

Don’t forget any of the string including the " and the -.

Note that this is not a curly quote, it’s the inch mark and the dash is a hyphen.

Now using the Widget Importer Exporter plugin, import that renamed .wie file you just found and replaced inside. Don’t worry: existing widgets will not be replaced, the only ones that will be ADDED are duplicates of the text widget but are using the new Classic Text widget. Yay, way faster than by hand and your content stays intact.

The final step is to toggle each sidebar/widetized area and either drag to inactive or delete the ones that are titled “Text: xxxx” and NOT “Classic Text Widget: xxx” or “Text” and NOT “Classic Text Widget” if you didn’t have titles in your widget.