Change product type.

You can use the WooCommerce bulk editor plugin, there are a few available and then you can edit things. It will be hard to do 20000 at a time but you can do it in 1000 products per batches.

Thank you for your response.

My understanding is that this feature is available in their premium or pro plugin. Is there a free alternative?

There are many free plugins available you can check here https://www.ads-software.com/plugins/search/WooCommerce+Bulk+Edit/

According to my previous post, I check every bulk edit plugin’s free version, but is this feature only available in the pro or premium versions?

Thank you

I am unable to answer that since I didn’t use all of those plugins.
you can test and explore the plugin features or ask plugin support if their free version has that feature or not.