How to update field in table FAGLFLEXA/FAGLFLEXT

Hi Gurus,
     we are stay in ECC 5.00. The new g/l have activated but we have not assigned the relevant scenario to the ledger (Refer note no 990612)
     now we have assinged following field:
     FIN_CCA: RCNTR (Cost center), SCNTR (Sender cost center)
     FIN_CONS: RASSC (Trading partner), RMVCT (Transaction Type)
     FIN_GSBER: RBUSA (Business area), SBUSA (Trading partner's business area)
     FIN_PCA: PRCTR (Profit Center), PPRCTR (Partner Profit Ctr)
     FIN_SEGM: SEGMENT (Segment), PSEGMENT (Partner segment)
     FIN_UKV: RFAREA (Functional Area), SFAREA (Partner Functional Area)
     our problem is the data was posted before that does not update fields in table FAGLFLEXA/FAGLFLEXT,How to update field in table FAGLFLEXA/FAGLFLEXT?

Hello Jenjira,
Since this was a question asked few years back, I think you can share your experience with activation of a Scenario subsequent to Go-Live of new GL functionality (We are contemplating activation of one more scenario for Functional Area updation in new GL. 4 update scenarios were activated at time of New GL Implementation 4 years ago).
Please share following:
1. Impact of this subsequent activation on Open documents in the live system.
2. How you addressed the issues arising on Point 1.
3. Any support that you got from SAP Support Team on issues thereof.
Any inputs will be of great help.