Thanks for your details..
999 is a special base pack (churn pack) provided for inactive customers as an incentive to recharge again. I am not quite sure about the interoperability between this pack and the My FTA pack. Likewise, the refundability is also relatively unknown in this scenario.
That said, if it were a normal DPO base pack, I am pretty sure it will work in the website/app. Please read my quoted post below:
Therefore, I would say you can try to change it. But at your own risk.
Or, you could probably call the sun direct nodal office between 10am to 5pm on a working day, and get their input before proceeding any further.
Pls provide nodal officer number for tamilnadu. I will check with them.