They sound as if they're old enough to start eating adult pellets, provided that you think they're at least six months old.
In order to condition them to new pellets, it's easiest to mix in a little bit of the new pellets with the old for a few days, then add a bit more, then add a bit more, and so on, until they are completely eating the new type of pellets. Some guinea pigs may require more or less time to get used to a change in their diet, and doing so drastically might impact their health.
When they are completely used to the new pellets, you can take the old ones away, but keep an eye on them to make sure that they're still eating.
Also, since they're getting bigger and becoming more of adults, it is usually recommended to lessen the amount of pellets they receive each day to 1/8 cup per guinea pig. You don't need to worry about this just yet, though. As long as they have 24/7 hay, it shouldn't be hard to get them down to 1/8 a cup per pig per day, when you do.