I think upon admission for your delivery, you should tell them. Upgrading from subsidised to b1/a1 is never a problem. But just note that afterwards eg. Clinic follow ups will be under private rates
Your gynae appointments. Since you’re considering to go private, you might wanna look for a gynae of your choice to look after you until you give birth. That’s the perk of being b1/a1 aka paying private rates.