Hi GF,
The Writings/NextHeir-anc won't change who have the existing Heir Apperent-trait, it only makes it likely that when the current leader dies, the guy with the ancillary takes over (and not the guy with the Heir Apperent-trait, unless they are the same). It's not definitive it will work, because of game mechanics, but it is likely.
It won't have an impact on Aragorn at all before he is adopted into the family, mind you.
Best of luck