When submitting a Cost Plan for Approval it returns an ALERT message;
ALERT: This cost plan cannot be merged with the existing budget because it has different grouping attributes or fiscal time periods. Proceed with this submission to replace the current budget, or submit a different cost plan to merge with the current budget.
CA PPM SaaS and CA PPM On Premise
In this scenario, since the current status of the budget is already set to APPROVED it cannot be updated anymore.
NOTE: Budget plan with APPROVED or REJECTED status cannot be updated any more.
As the ALERT message indicates you can proceed with this submission to REPLACE the current budge or SUBMIT a different cost plan to merge with the current budge.