Parallel conversion operates both the old and the new system for a limited time. Phased conversion changes to the new system in phases, introducing some of the new applications while still using some of the old applications, or converting some departments or locations at a time. Pilot conversion lets one department or location try out the new system, while the rest of the organization continues with the old system for a while. Plunge converts the whole organization to the complete new system at one time.
Which conversion strategy is best depends on the specific application and environment. Parallel conversion poses the least risk, but is also very expensive. Plunge may be the cheapest, but entails the greatest risk.