old account records

Pls let me know.

public class HelloWorld
{
   public void Display(List<Account> acc)
    {
       for(Account val:acc)
       {
           if (val.Hello__c == null)
                 val.Hello__c='World';
       }
    }
} 
trigger trgupd_hello on Account (before Insert,before update,before delete,after insert,after update,after delete,after undelete)
{
      if (trigger.isBefore) 
      {
           if (trigger.isInsert)
           {
               HelloWorld obj_hello=new HelloWorld();
               obj_hello.Display(trigger.new);  
            }
     } 
}