O2 prepaid to O2 Handyvertrag with same number

Hallo! I am planning on buying a Handyvertrag with an O2 SIM from the Samsung website. I currently have an O2 prepaid SIM. I am assuming the contract comes with a SIM with a different number. How do I go about the transition while still retaining my same (old) number? Will the process be the same if I transition to a Vodafone Handyvertrag instead? Danke!