Fields such as schedule % complete, planned value costs, etc. that use a baseline to help determine the value depend on the activity id's to match between the baseline project and the current project. If the activity id is changed the link is broken. In the P6 power client there is an internal guid that allows you to change the activity id in either the baseline or current project and still have these values related. In the P6 Reporting Database the activity id is used as the joining characteristic between which activities are a match between a baseline project and a current project.