For VP9/x264, almost certainly not. If you jump on a newly-uploaded video, you'll see that higher resolution comes later. It's common to see 720p nearly immedia

For VP9/x264, almost certainly not. If you jump on a newly-uploaded video, you'll see that higher resolution comes later. It's common to see 720p nearly immediately, then 1080p, then 4K. For pre-x264, they probably could, but between the relatively small sizes required for the low resolution those codecs would be supporting, and the cost difference between compute and storage, I'd bet everything is encoded beforehand.