.NET SDK - upload file with versioning

Using the Box .NET SDK.

Am I doing this correctly?  I want to upload a file and if it already exists in the account, upload a new version.  I'm doing a "preflight check" to see if the file already exists.  If it doesn't, I can just upload the file.  If it does exist, then I get the BoxFile.Id of the existing file and use that in my BoxFileRequest.  My assumption is including the file Id, Box will assume I want a new version.

BoxFileRequest request = new BoxFileRequest()
{
    Name = fileName,
    Parent = new BoxRequestEntity() { Id = boxFolderId },
    Id = fileId
};

result = await this._boxClient.FilesManager.UploadAsync(request, stream);

However -- I'm still getting a "file name in use" exception.  Am I doing something wrong or is the problem I'm using a free Developer account and maybe versioning is not allowed.  Also, I'm uploading the exact file again with no changes.

If you need more info, please let me know.  Thanks!