For the first week at least,, sometimes two, it's recommended to stick to the same exact diet the baby bun is used to, unless it's causing serious digestive illness.
Baby rabbits have a delicate digestive system, and the stress of a new home can affect digestive function. So adding a change of food to that isn't a good idea. Usually the place you get the bunny from will give at least two weeks of pellets so you can transition your rabbit to the pellet you plan on feeding. If not, find out the exact same type and brand of pellets they feed, and get those to start with. After the bunny has had a week to settle in, if you are going to change pellet food types, a slow transition can usually be started after that first week. It should take two weeks to completely transition from one pellet type to the other, provided no mushy poop or digestive issues develop.
Like mentioned, alfalfa hay isn't necessary when alfalfa based pellets are fed. Feeding alfalfa hay in addition can actually be too rich of a diet, causing slower digestive function, excessive production of cecotropes that get left uneaten, and will usually make for a very picky rabbit that doesn't want to switch to a grass hay when they reach adulthood.
Alfalfa based pellets limited enough so that bun is eating a pile of good quality grass hay the size of their body per day, is what I prefer to do for baby rabbits. Then monitor droppings and weight/body condition to see if this diet is working well for your rabbit. There should be no uneaten cecals, no mushy poop, and normal round golden fecal poop. Body condition should be rounded and not bony along the back and hindquarters.
baby rabbit diet