They won’t be illegal to own, just to manufacture new ones. The market for them will dry up by attrition and hoarding, especially the standard cap magazines.
After the first AWB in '94, some clever people were selling 10 rd mags that were the same dimensions as 30 rd mags. There was a little metal tab that prevented the feed chute from accepting more than 10 rounds. All it took was a screwdriver to pry that tab out and you now had a fully-functional 30 round magazine.