Add new field to a MongoDB document parsing from String to Int using updateMany

In my MongoDB collection, all documents contain a mileage field which currently is a string. Using PHP, I'd like to add a second field which contains the same content, but as an integer value. Questions like How to change the type of a field? contain custom MongoDB code which I don't want to run using PHP, and questions like mongodb php Strings to float values retrieve all documents and loop over them.

Is there any way to use \MongoDB\Operation\UpdateMany for this, as this would put all the work to the database level? I've already tried this for static values (like: add the same string to all documents), but struggle with getting the data to be inserted from the collection itself.

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