Research is all about discovery, and the process of doing research sometimes requires you to shift gears and revise your intended path.
Your DMP is a living document that you may need to alter as the course of your research changes. Remember that any time your research plans change, you should review your DMP to make sure that it still meets your needs.
Data management is best addressed in the early stages of a research project, but it is never too late to develop a data management plan.