Depending on the tool(s) you are using to track and manage your plan, the revisions may be tracked by the software. If there are Word doc plans, Word can track all changes. If you upload the Word doc to SharePoint, SharePoint can track the versioning. If you are using MS Project, you can track when a new baseline was created. Worst case, just develop the discipline that each plan change is saved as a separate file with the date of update incorporated into the file name.