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Yes. We can change it any time.
Last month, I was trying out (in the mobile website) to see if a base pack change works. I accidentally ended up changing my base pack from DPO (yearly pack) to NCF (monthly pack). It got changed immediately to NCF (for 1 month), and the remaining money was refunded to my sun direct wallet. I changed it back after a couple of hours (from NCF to DPO). It worked again.