You can directly change the Budget at Completion (BAC) of a WBS element even though the element has work-in-progress milestones that have already been claimed. However, be sure you want to change a WBS element's BAC, as the change will affect the BCWP for that element.
Because the BCWP of a WBS element is stored as a percentage of the BAC, changing a WBS element's BAC value will affect the BCWP of all claimed milestones to date for that WBS.
How previous BCWP values are affected depends on the EVM used.
When you change the BAC, and BCWP is recalculated, the percent complete for each month remains constant but the value it represents will change. For all months that you compute BCWP, the BCWP values will change to reflect the stored percentages of the new BAC.
Because an EVM of BCWP Entry or Earned Standards requires you to directly enter BCWP values, these values are saved as discreet values, not as a percentage of the total BAC.
If you increase the BAC for a WBS element and recalculate the BCWP, the direct entry value will not be changed (providing the base unit has not changed and BAC was not reduced), and the percent complete for each month is recalculated based on:
BCWP in base unit / BAC in base unit
However, if the BAC was reduced, BCWP is recalculated to avoid the BCWP at the end of the project being greater than the BAC.
This also works for Earned Value Method Standard Hours entry. Entry of Cum BCWP in standard hours results in a calculated percent complete, which is then applied to both BAC of Hours and the Cum BCWP (which is stored in Hours rather than Standard Hours).
Prior month BCWP values and BCWP percent complete will not be modified if you change a WBS element's BAC. Any changes to BAC will not be reflected in prior month calculations.
Prior month percent complete values will always represent percent complete of BAC at the time that month was calculated.
Modifying an existing estimate after statusing the WBS will impact the calculation of BCWP values.
Modifying Milestone Information for Work in Progress