Is that strategy possible at all? Are there cases tested in courts (both US and EU) that assert that the file-by-file licence replacement really works?
Why wouldn't it? The author of the file will be the copyright holder by the virtue of authoring it. If she does not get rid of it, she still remains the copyright holder. Copyright does not switch owners purely by the virtue of distribution with another piece of work.
The copyright holders can later on relicense the work under something else. Licensing a work under some non-exclusive license does not restrict you from licensing it again in the future under other terms.
