How to add a new product to an already activated physical SIM via Connect.
In order to add additional plan on a physical SIM, after the initial activation, use the order API. The order request will include the ID of the product that will be activated via the order, check the Product Catalogue guide in order to understand how to access the product catalogue. For physical SIMs (pSIM) the order also needs to reference which iccid will be activated in the order.
Order fulfilment is not synchronous, so after placing an order, the order status needs to be periodically checked until completion using a polling mechanism. For this your code can use the order API or just use the metadata links available in the response from order creation. You can fin the full list of order status here.