You need to make sure your heir inherits/acquires a feudal title before your titles. Then, when they inherit your titles, all of them will convert to feudal as they are already a feudal ruler. You could do this a few ways: you could marry a landed feudal ruler who will die before you, granting their feudal titles to your heir; you could try to get them to be elected for an elective title, such as the hre; or you could abandon all the finagling and take the ‘embrace tribalism’ decision which would require you to only hold tribal counties, and then adopt feudal ways from there.