Let us know how you get on at the vets. It is notoriously difficult to accurately sex bunnies, especially ones as young as yours. We were convinced Bramble was a girl until he was almost 20 weeks old. You don’t want to take the chance of an accidental litter (which is actually how we got Bramble in the first place) and buns can get pregnant very young, if you have two same sex buns they will start to fight if left unaltered so best to separate them until their spay/neuter and recovery time is over.
When it comes to changing their food at 12 weeks you will want to mix the old and new food together and phase out the old food, then work on reducing the amount in line with their weight and the addition of veggies. Start with one veg at a time and only small amounts, you can increase the amount a little each day and add a new veg every few days as long as there is no tummy trouble. There are plenty of posts on here about rabbit safe veggies, I am quite lazy and buy bagged salad for my two I just have to watch what it contains and I top it up with a few other bits and bobs.