Ground Rule Doubles with Runners on Base: Understanding the Rules

Question
What are the rules regarding base runners already on base when a ground rule double is ruled.  I had an umpire tell me that if there is a runner on third and only on third and somebody its a ground rule double that the runner on third doesn't move cuz he wasn't forced.  Is that correct?  He also said then that if there was a runner on second he would only move to third.  I thought everyone was awarded 2 bases.  What is the deal

Answer
Hi Chris,

I had an umpire tell me that if there is a runner on third and only on third and somebody its a ground rule double that the runner on third doesn't move cuz he wasn't forced.  Is that correct?  
NO that's not correct.  All runners are awarded 2 bases from the time of the pitch.  8-5-I

Mark