The dynamo lights are meant to be interchangeable. The front lights take 6V/2.5W and rear 6V/0.5W or less. Halogen is only used for front. There is only one subtlety regarding halogen in that they are less tolerant to overloading than standard incandescent. A good halogen will have a protection built in, but cheap might not. If you ride a cheap front halogen off a standard 3W dynamo and do not connect a rear lamp or connect an LED rear lamp, you may blow the front when at high speed.