I don't think it's possible to change Recipient Type using the API. From page 159 of the DocuSign REST API Guide (https://10226ec94e53f4ca538f-0035e62ac0d194a46695a3b225d72cc8.ssl.cf2.rackcdn.com/rest-api-guide-v2.pdf):
Once an envelope has been sent, you can only edit: email, userName, signerName, routingOrder, faxNumber, and deliveryMethod.
So, changing a recipient's type would likely be a two-step process with the API:
ADD recipient (with new type) to the envelope:
POST /accounts/{accountId}/envelopes/{envelopeId}/recipients
DELETE recipient (with old/original type) from the envelope:
DELETE /accounts/{accountId}/envelopes/{envelopeId}/recipients
I'd suggest doing things in this order (adding first, then deleting) so that the Envelope won't advance (to future recipients) in the workflow until that recipient completes their required action.