If you have an on-prem Shotgun install you should still write in so we can switch things off for you. You should then also take the transcoder workers offline as they’d just be consuming resources for nothing and would never be called upon.
As for uploads… Uploads and uploading is a very generic process and will continue to work as it usually does even with DIY transcoding. Uploads to the sg_uploaded_movie field on the Version entity is the magic bit which triggers transcoding. If you opt for DIY transcoding, uploading to that field would work but would just not trigger the transcoding process. You’d have to upload your own media to the sg_uploaded_movie_mp4 and sg_uploaded_movie_webm fields as well as populating sg_uploaded_movie_frame_rate.
I sure hope this helps answer your questions.