I have now upgraded Netflix from Basic to Premium.
The difference amount was as expected. Rs.300/month (no additional GST).
Interestingly though, I had to make the payment to Jio, and not to Netflix as I had expected.
The payment was requested for the whole validity period. Meaning, until the Jio Fiber's 1499 plan ends for me (13 months).
So I had to pay Rs.3870 for 13 months of Netflix Premium.
I had recharged the 1499 plan a few days back (Aug 27). Thus, the payment amount was reduced by Rs.30.
In case if I had done the upgrade on Aug 27th itself, the payment amount would have been the full Rs.3900.