A first theoretical study on the possibility for stepwise tuning of the frequency of the KIT 170/204 GHz TE34,19/TE40,23-mode pre-prototype has been done. A bandwidth of ± 10 GHz around both center frequencies is considered as tuning range. For each of the two bands in total 11 modes have been selected to cover the entire frequency range. The theoretical study has shown that just changing the azimuthal index of the mode series is insufficient because the insert loading constraint is not fulfilled. A new mode series has been found in which the TE42,22-mode is excited after the TE40,23-mode and reduces the insert loading to a maximum of 0.1 kW/cm2.