Upgrade .doc to .docx in-place in a doc library

using Microsoft.SharePoint.Client; 
.... 
using(var ctx = new ClientContext("http://yoursharepoint/web_containing_library/")) 
{     
var lib = ctx.Web.Lists.GetByTitle("Title of your library");     
var files = lib.RootFolder.Files;     
ctx.Load(files);     
ctx.ExecuteQuery();     
foreach(var file in files)     
{         
if(file.Name.ToLower().EndsWith(".doc")         
{             
string newFilename = file.Name + "x"; //dirty hack :)             
file.MoveTo(newFilename, MoveOperations.Overwrite);             
ctx.ExecuteQuery();             
//not sure if you need this, try without first             
ctx.Load(file);             
ctx.ExecuteQuery();            
 //convert DOC content to DOCX             
var fi = File.OpenBinaryDirect(ctx, file.ServerRelativeUrl);             
//get content using fi.Stream and convert it             
//save it             
File.SaveBinaryDirect(ctx, file.ServerRelativeUrl, streamWithDOCX, true);         
}    
}
}