I'm not 100% sure I understand this:
If you rename the newly uploaded file to the same as the original file name, you are overwriting the original file. The original file no longer exists on the server to be deleted. in this case there is no need to use a delete file server behavior. You are "deleting" the original file by overwriting it.
If you are not renaming the new file to be the same name as the origianl file, you can use the delete file server behavior to delete the original file, just reference the hidden form element that contains the default file value.