I'm not sure about the Salesforce part of this question, but in regular DocuSign, depending on how your account is configured, the change signer option may be either OFF for your account or OFF by default but capable of being turned on for the envelope. When sending, if you click on Advanced Options, check to see if the option is available there. If it's not available there, you may be able to enable the feature in your Settings Tab on the Signing Settings page.

Thanks a ton! It was in the Settings Tab. I had tried that setting individually on the envelopes, but I couldn't get it to work. Once I set it in Settings though, it's working.

I am having the same issue. How do I get to the Admin Console? Can't seem to figure it out.