Maybe instead of all of them, just the IDs for hair, facial hair & head types not in FIFA16? That would save time & avoid unnecessary duplication.
They replaced some old hair models like ID 36 or 53 over the time as far as I know. Just compare the Fifa 16 ones with
those of Fifa 19 and you will see some new hair models with old IDs. And I'm not sure if the converted ones would be slightly bigger or smaller than the original ones. Therefore converting the whole set would make more sense in my opinion to avoid issues when old heads aren't compatible with new hairs or the other way round.
Here are also ID Lists of
Fifa 20 and
21, but they just show "hidden hairstyles" whatever that means in that regard.