Hi Nars,
Assuming, that you have to update only the document no. and the document item and the selection values remain constant for all the incorrect items, i.e. G (Business Partner)-Sel. category 1, B(Document No.)-Sel. category 2 and H (Document Item)-Sel. category 3, then technically you have to update two database table-DFKKZP and DFKKCFZST, with the new values of document no and document item.in the Selection value 2 and Selection value 3 respectively.
As suggested by Bill, SHDB is not an option.
Also as per my knowledge goes, any changes in the clarification item is being handled by series of standard include programs that gets called during the save .
I would suggest you to try the FM, ISU_CFPS_CFC_EVENT_CF_FILL, where I_APPL_OBJ=PLOT and I_APPLICATION_ITEM=payment lot item.
If its possible through this FM, then its fine, other wise the last option that remains is to build a table update program to rectify the incorrect items.
Hope it helps.
Thanks,
Amlan