code: func (c *Client) AddonUpdate(appIdentity string, addonIdentity string, plan string) (*Addon, error) {
  	params := struct {
  		Plan string `json:"plan"`
  	}{
  		Plan: plan,
  	}
  	var addonRes Addon
  	return &addonRes, c.Patch(&addonRes, "/apps/"+appIdentity+"/addons/"+addonIdentity, params)
  }
docs: // Change add-on plan. Some add-ons may not support changing plans. In that
  // case, an error will be returned.
  //
  // appIdentity is the unique identifier of the Addon's App. addonIdentity is the
  // unique identifier of the Addon. plan is the unique identifier of this plan or
  // unique name of this plan.