PayPal - Updating Redirect Urls

Question

I have initiated a Paypal Payment, got a token back. PayPal has redirected back to my site using the URL I set in the Payment. From here I get the users shipping address, update the shipping costs and wait for the user to confirm payment. However, at the same time (but in a separate function) I want to update the redirect urls (actually only the return_url if that's possible). I have the following code:

RedirectUrls redirUrls = new RedirectUrls()
{
    cancel_url = ppmodel.CancelUrl + "&cancel=true",
    return_url = ppmodel.ReturnUrl + "?guid=" + basket.OrderGuid
};


PatchRequest pRequest = new PatchRequest();

Patch pUrls = new Patch
{
    op = "replace",
    path = "/redirect_urls",
    value = redirUrls
};
pRequest.Add(pUrls);
payment.Update(_apiContext, pRequest); --Error here

I am getting a server error 400 Bad Request. Can anyone tell me what I am doing wrong?