One of the signers in my envelope is on extended leave. How do I change my envelope to reflect his delegate without resending it to everyone else who has already signed?

The best option, if you are the Sender of the envelope, is to perform a "Correct", which will allow you to change the Recipient name/email address to the delegate and have them notified they are next to take action. This should allow a continuation of the flow and would not affect previous signers. Signers who already completed should not be notified of this change rather they would still get a completion email at the end of the entire process.