There are several ways to approach this.
Yes, you usually can reuse the keepers.
On low mileage engines you can often get away with replacing the bad valves. You do need to check the valve seats and make sure they are not damaged. At the least, you should get a valve stick and lap in the new valve to the old seat and confirm that the seat is OK.
The more common approach is to take the head to a machine shop and have them grind all the valves and seats. If you have access to valve grinding equipment (auto shop at school) you may be able to tackle this yourself.
Replacing all the valves is seldom necessary as well as very expensive.
