Replace the currently used old DICOMSeriesReader in org.mitk.gui.qt.dicom and use the infrastructure given by the DICOMReader module.
I would propose to directly switch in this plugin and not over old and new as optoin (like we do currently (T18289) in the general loading mechanism of the workbench. Reason: the potential changes in behaviour are small and as far as I know the plugin is only rarely used currently. So when deciding between usablility (no extra selction in an option box) and definsive mirgration strategy, i would opt for the first one.