Keep in mind that you only need to provide ONE source of extra calcium. So there is no need to buy young guinea pig pellets, alfalfa hay, AND provide high calcium vegetables. Also if you chose to give alfalfa hay you still need to provide timothy hay, at a ratio of roughly 70% timothy to 30% alfalfa, as alfalfa hay is a legume hay it doesn't keep their teeth ground down as well as a grass hay like timothy does.
If you chose to feed young guinea pig pellets that is the only thing you need to change, if you want to feed alfalfa hay (with timothy hay) then you can still feed adult guinea pig pellets. If you want to add a high calcium vegetable to their daily diet you can feed adult guinea pig pellets and just timothy hay.