I would like to know if it’s possible to transfer an HD-DVD 30 GB movie onto a BD-R 25 GB disc and just removing the extra features that are on the HD-DVD or would transcoding still be needed in order to fit on a BD-R 25 GB?
Yes it is possible.
But a lot of work is needed using some software/tools available online.
Note that HD DVD video frame rate is 29.97 FPS while Blu-ray 23.976 FPS so we will need to convert/patch the video. Also many HD DVD audio uses DD+ (Dolby Digital Plus) where Blu-ray does not recognize it, so you also need to convert DD+ to DD.