Hey @kaiwenlim, thanks for reaching out to the Community!
I understand your concern.
Your new payment date will likely be a few days earlier than usual to cover the price difference between Individual and Premium Duo.
My recommendation if you still want your billing date to be on the first is to cancel your Premium, wait for your account to revert to Free - and then subscribe to Premium Duo on the date you want to be charged for the plan (that will be your new billing date).
More information on Premium Duo can be found at this Spotify Answer.
I hope this helps!