Are you wanting to breastfeed or just stop using formula and give breastmilk in a bottle?
If you're wanting to breastfeed, at that age I would keep baby with you 24/7 using a baby carrier if necessary (can be a Godsend when you have other children!) and just stop using bottles as cold turkey as possible (I'd watch output and if it dips, give some milk in a bottle but keep it to the minimum). What you can express doesn't mean much; I've never been able to express more than 2 ounces and my LO is 2 now and still going strong on the boob. A baby is always going to be miles better at getting the milk and stimulating supply than even the fanciest pump so giving him/her unrestricted access to the boob is the best way. If you're wanting to use the pump, I would use it more often than every 2-3 hours as a BF young baby will often want to feed more often than that. Don't skip the night time either, in fact the night is the best time to boost your supply.