Changing one datatype to another in all columns of SQL Server in Java

Question

I am using Altova XMLSpy to create database from one GML schema. But, all string type fields are converted as char(255). I need to make it nvarchar(255). Can I do this in Java and how? I think it would be usefull because I need to make more than one DB at this way, so I need to automate process. Or, (I know it is not the question's title) can I override xsd:String to something else that XMLSpy wolud recognize as nvarchar(255)?

P.S. XMLSpy offers me on export to I manualy change all of this, but column by column.

EDIT: This DB is empty, no records at all.

EDIT2: I solved my problem, partly. Now, it changes some of columns but not all. Here is my code. Where I am wrong?

public static void main(String[] args){
    DataTypeChanger test=new DataTypeChanger();
    test.connect();
    test.dropCheckConstraints(); // it drops them successfully, I don't need them.
    test.disableConstraints();
    test.changeDataType();
    //test.enableConstraints();

    try {
        /*test.st.close();
        test.rs.close();*/
        test.conn.close();
        System.out.println("Disconnected!");
    } catch (SQLException e) {
        e.printStackTrace();
    }

}

public void disableConstraints(){
    try {
        st = conn.createStatement();
        st.execute("EXEC sp_msforeachtable \"ALTER TABLE ? NOCHECK CONSTRAINT all\"");
    } catch (SQLException e) {
        e.printStackTrace();
        System.out.println("Query ne radi!");
    }
}

public void enableConstraints(){
    try {
        st = conn.createStatement();
        st.execute("exec sp_msforeachtable @command1=\"print '?'\", @command2=\"ALTER TABLE ? WITH CHECK CHECK CONSTRAINT all\"");
    } catch (SQLException e) {
        e.printStackTrace();
        System.out.println("Query ne radi!");
    }
}

public void changeDataType(){
    try {
        st = conn.createStatement();
        Statement stt = conn.createStatement();
        rs = st.executeQuery("SELECT a.name as ColName, o.name AS TableName FROM sys.syscolumns AS a INNER JOIN sys.systypes AS b ON a.xtype = b.xtype AND b.name = 'char' AND a.length = 255 INNER JOIN sys.objects AS o ON a.id = o.object_id WHERE (o.type = 'u') AND (o.schema_id = 1)");
        while (rs.next()){
            String tName=rs.getString("TableName");
            String cName=rs.getString("ColName");
            //System.out.println(tName+"  "+cName);
            try{
                stt.execute("ALTER TABLE "+ tName+" ALTER COLUMN "+cName+" nvarchar(255)");
            }catch(SQLException ee){

            }
        }
        stt.close();
    } catch (SQLException e) {
        e.printStackTrace();
        System.out.println("Query ne radi!");
    }
}