hi eric! that rule would transform into the point reallocation rule in the different versions of basic d&d, but in the original ruleset it is implied that there is no actual reallocation :)
for example, suppose that a fighter has 12 strength, 11 intelligence, and 12 wisdom. they have 11 - 9 = 2 points of intelligence which can count as 1 point of strength, and 12 - 9 = 3 points of wisdom which can also count as 1 point of strength. therefore their overall total “strength” for the purposes of XP is 12 + 1 + 1 = 14!
another example is that if your character has 10 strength, 15 intelligence, and 15 wisdom, the math works out so that they can also be a fighter with a virtual PR score of 10 + 3 + 2 = 15!
meanwhile, all the scores themselves seem to stay the same.