Certain BSEG fields, which could be changed, If you still want to change such fields with FB02 (and thus, have different values in the FI document and in the FI-SL document after the change), you can use the following trick:
Change the field movement in which you use the field in question (for example, SGTXT) as follows (Transaction GCF2):
Instead of sender field SGTXT, enter another field (that you do not use and in which you are not interested at all), for example ZTERM. In addition, enter an exit Uxx (xx being a two-digit number that you do not yet use) in the 'Exit' field of the same line. Save. Then, in this exit, instead of the FROM_FIELD, move the field that you are really interested in (in our example SGTXT) to field TO_FIELD: 'MOVE ACCIT_GLX-SGTXT to TO_FIELD.'
For more details on how you create and write user exits for FI-SL field movements, refer to the F1 documentation on the 'Exit' field in Transaction GCF2. You can also take a look at report RGIVS000 (SAP Exits for Variable Field Movements) as a source code example.