For the first time in my career I encountered a video file that had two different resolutions. The beginning of the video was SD resolution and after 15 frames it jumped to a resolution of 1080p. VLC player was able to correctly switch the resolution during playback and it displayed the two different resolutions also in the codec information window. But Premiere Pro didn’t understand the file properly and never switched to the higher resolution portion of it. That was a problem because I wanted to use the high resolution in my edit.
It turns out there was a relatively simple solution. The file was encoded as a motion jpg and ffmpeg was able to convert it to an image sequence without re-encoding it.
The command was written by user martjno at Stack Overflow, thanks to him for this:
ffmpeg -i mjpegvideo.avi -vcodec copy frame%d.jpg
You need to download ffmpeg, open the command line window to that location and the run the command (just replace mjpegvideo.avi with the path to your video file). It will create a jpeg picture for each frame, using the correct resolution.