Leveraging partial baseline feature...

Change is inevitable. Adding new tasks to a project that has already been approved and baselined can be troublesome—to you and stakeholders. But don’t fret. You don’t need to set a new baseline to take into account the new tasks. Just update the old baseline. To modify the baseline that has already been set, do the following:

1. Select the new tasks that have been added recently to the project. This is important.
2. On the Tools menu, point to Tracking, and click Set Baseline.
3. In the Set baseline list, select the previously set baseline. It will have a date associated with it.
4. Here’s the tricky part. Make sure you select the Selected Tasks option. Otherwise, you’ll reset the baseline for the entire project.
5. Click OK.

Source: MPUG