How good is your data? How well does it meet the requirements of your new ERP system? How much of the cleansing/migration effort can be automated and how much is the business going to have to do manually? Unless you know the answer to these questions it is really hard to plan a data migration project well.
This is why the first phase of in a data migration methodology should be to profile the data. The tools I prefer to use to do this are normally SAP BusinessObjects Data Insight and Data Services.
These tools, along with a basic knowledge of the requirements allow the development of a snapshot of the data in the legacy system which can provide “insight” into how well it will fit into the new system, what issues might be coming up and also provide an idea on whether the problems can be fixed programmatically or if they require that human touch.
All too often, at the last minute, the business is told that there is data missing and the new system won’t work without it. They then run around hectically trying to get good accurate data or just throw their hands in the air and provide ‘default’ data just to get something into the system. Neither option is best. By doing profiling as the first phase of the project many of these sorts of situations can be identified early and avoided. The business can get sufficient time to be able to collect the required data and to ensure it meets the intended business needs in the new system.