SPI/CPI and Project Change

Hi

I understand how to calculate CPI and SPI and use them in communicating project status. However, can anyone tell me what would happen to these indicators if a project change was raised to alter the scope of delivery, which in turn impacted the time and cost to the project?

Would the project have to be re-baselined? if so, how does this affect the month-on-month comparison of the numbers?

If the project isn't re-baelined, then again, how does this affect the results?

Thanks in advance...