I just googled a bit to see how much space could be saved by converting Blu-ray AVC content (1080p non-HDR) to AV1 with subjectively same-ish quality, and from what I gathered one can expect to save 40-50% storage space on average (with a spread of maybe more like 20-80% depending on the video). So it’s substantial, but maybe not substantial enough to run and reencode all your movies, unless moving to a free codec is the important motivation.