Changing the slice view orientation just changes how the image is displayed and will not modify the image.
You can reslice the volume, for example using Crop volume module. You can enable rotation of the cropping region (ROI) in Markups module:
Why would you like to export the resliced image to DICOM? Would you like to do some additional processing in some other software? Why would you like to export to DICOM format instead of a simpler research format, such as NRRD?