Normally Delta will switch your flights around to accomodate you due to this type of schedule change. If the first agent refuses to do it, hang up and call back. They could potentially refer you back to Continental since it was a Continental issued reservation/ticket. With you having to deal with both JFK & ATL potential messes, I'd allow at least a 3 hour connection in ATL though, otherwise you could be at a huge risk for misconnection.
