The recommended way to move files is to remove them from one study and add them to the other via the user interface.
If you have configured your own DVN, it may be possible to make the change directly in the database but this is a fairly complicated process and not recommended. There are both database and filesystem components that need changing. Additionally, our application's study versioning policy and design currently preserves the state of previous study versions including any associated files, so while they may be removed from the currently released version, they would still exist and be accessible in an archived version.
We are looking into improving the behavior of "deleted" files by preserving the file name associated with an archived version but removing the actual file.
Let us know if you have further questions or concerns.