Can you remove the `translatable` option to a matrix field without losing locale content?
This isn't a direct answer but one possible path if an easier option doesn't surface. 

Sprout Import supports importing content into a Matrix field and multiple imports can be created to handle multiple locales while still matching things back to the parent entry.


Example of importing content to a Matrix field
Example of importing content for multiple locales
Example of how to generate a JSON Import file in your Craft templates. While this example uses fake data, you could update the loop to output the content you need to re-import.


It's likely you'll have a handful of relationships in your content that you'll need to make sure resolve correctly as well if you re-import your content, so depending on how your content is set up there may be a few more tricky parts to work through. But this is one possible path if other options seem like the may require a lot of manual steps.

Be sure to test this in a development environment and make the appropriate backups before modifying any live content.