Hi Ritesh,
The change log for CJ40 Unit cost planning screen that we can see is the standard SAP functionality. As you can see in Table CKHS for Unit costing header data, the table line item has created by/on and change by/on details which is also displayed in the CJ40 Unit costing dialog.
If the complete log of details for CJ40 Unit cost planning changes is required, then you might need to create a Z-table that should be filled by enhancement spot of CJ40 (I believe this to be ES_SAPLKKDI) whenever any changes are done in Unit costing sheet.
Hope this helps.
Best regards,
Aditya