I guess there is same bug with rental invoices: will be there wrong links depends on whether the new inoice number will be higher or lower than the old one, but I do not have rental configured and cannot check.

I will answer myself:

//proper way of updating one-to-many linked records ("on update cascade" in SQL terms)

FuelSDrsr.SDNr=oldNr;
While ReadFirstKey("SDNr",FuelSDrsr,1,true) begin
RecordCopy(oldFuelSDrsr,FuelSDrsr);
FuelSDrsr.SDNr=newNr;
if (RecordUpdate(oldFuelSDrsr,FuelSDrsr,false)==0) then begin end;
ResetLoop(FuelSDrsr);//Is it necessary?
FuelSDrsr.SDNr=oldNr;
end;