Would creating an Amazon ELB affect running instances?

I have several Amazon instances running my company's websites in the default VPC. I want to create an Internet-facing load balancer and add new instances under it for the given VPC.

My question is would the creation of a new ELB affect request routing to my old instances unless I add them explicitly.

I am afraid that my websites would go off-line in the case I misconfigure ELB.