It's common that that there are changes in the DL network. Schedule changes, equipment changes and flight number changes are not uncommon. So the KLxxxx flight nr gets a DLxxxx flight number because you are technically moved to a new flight even though the STD, STA, equipment etc is the same. The system moves you then to a DL nr and not a KL number. For many flights, earnings on a DL number are higher in the FB scheme, so I didnt complain when I had the same situation last summer.