You will still be able to watch the prepaid channel until the end of its validity even if the HD Pack has expired. However, you will not be able to purchase any prepaid packs or channels without the NJOI HD pack active.
Below is an example to illustrate this.
Customer A bought both NJOI HD pack and Astro Ria on their respective dates and NJOI HD expires 4 days earlier than Astro Ria. Customer A will still be able to watch Astro Ria up until 5 July, despite NJOI HD pack expiring earlier.