Prepaid sim fees after expiry?

Hi! I got a prepaid CMHK 7-day ($78) sim card on a trip to HK a while ago, and refilled it when it expired so it is alive until September with money i the account. Can I make calls and use cellular data without subscribing to a plan? If so, what is the rate for these services? Am I able to switch my prepaid sim to a “normal” sim card just by purchasing a subscription plan? Thanks