In the quest to stay ever current of technology, companies constantly face the challenge of deciding whether to further complicate old, existing systems, or whether to scrap the old technology entirely for something new. On first inspection it is often difficult to appreciate the extent of effort that has been invested in a legacy system. Whereas on the surface the risk here would seem to be a straightforward analysis of uncertainty versus potential reward, the deeper diagnosis actually revolves around customer service issues.
Most likely a high percentage of your existing customers use only a small contained core section of your legacy system. You want to target the 70 percent or so of these customers who you can move to the new system with the least amount of risk. Realistically however you should prepare both the management and you staff that the process will essentially give your revenue a 30% haircut: it is simply unprofitable to convert everyone away from the legacy system, as after the new system is close to code-complete it becomes too costly (primarily for your customer support and technical support staffs) to maintain the other 30% on the legacy.