Hi Yashpal,
I'm afraid if dependent data (actual and/or planning data) exist for cost element for the fiscal year, then there is no way to change the category type until the next fiscal year. This is to prevent inconsistency in the data for a given fiscal year. When posting the first transaction data (plan or actual posting), a year dependent entry is created in a control table for the cost element.
You can only change the category if NO transaction data exist for the relevant year or if all the transaction data for this cost element/year is deleted.
An option is to create a new analysis period (KA02 -> Basic Screen -> Edit -> Analysis Period) and specify the new cost element category.
The new analysis period can only be specified in yearly intervals.
For further information on this review the note:
37093 Master data: change/delete validity period
https://css.wdf.sap.corp/sap/support/notes/37093
Hope this helps.
Regards,
Hugo