Newbie question: how to change netcdf header?

Hi,

I'm new in netcdf Java and I'm wondering if it's possible to change the 
netcdf header without changing the data. I've tried to do that but it 
doesn't work (v2.2.10 API):

-------------------------------------
import java.io.*;

import ucar.ma2.*;
import ucar.nc2.*;

public class test2 {

 public static void main(String[] args) {
   String filename="/home/netcdf/foo.nc";
   try {

     //Load netcdf:
     NetcdfFileWriteable ncfile=new NetcdfFileWriteable(filename);
    
     //Modify a netcdf variable (lattitude): from float datatype to 
double datatype      
     Variable varLat=ncfile.findVariable("lat");
     varLat.setDataType(DataType.DOUBLE);//lat from float to double
    
     //ncfile.create(); -> doesn't work
    
     //Close:
     ncfile.flush();
     ncfile.close();

   } catch (IOException ioe){
     ioe.printStackTrace();
     System.err.println("ERROR creating file");
   }
-------------------------------------

I think that I'm missing a method that actally writes the changes to the 
nc file but I don't know wich is it. I've tried ncfile.create() but I've 
got "not in define mode".

Any help would be appreciate.

Thanks in advance,

Roger Olivella