So Spake Martin
GRRM touched on this subject in correspondence with a fan.
Tigers14: another question, can NW vows be annulled if a person had no idea who he really was when he took them?
GeoRR: who had no idea who he was?
Tigers14: jon
GeoRR: Jon knows who he is. He may not know who his mother is, but that's not the same thing. There are plenty of orphans and
bastards in the Watch who don't know who their parents are.
Tigers14: yes. but if jon is the legitimate son of rhaegar and lyanna , he is the king of westeros.
GeoRR: well, you know I am not going to get into any of that
GeoRR: I think I've said enough for tonight.
So while George R.R. Martin seems to imply that it doesn't make a different (Or at least doesn't say that it makes a difference), he also refused to comment on Jon's case. So I believe we can say that George is pretty much vague on the subject.
Speculation
While there aren't many direct examples of similar cases from the Night's Watch, we do have two If-ish examples from the Order of the Maesters, who swear similar vows of renouncing all claims to land, title, wealth and are sworn to serve rather than lead. Do note, none of these people:
- Took the vows while simultaneously being heirs to something.
- The only way they became heirs eventually was all about luck.
First (weaker) example would be Prince Vaegon, third son of King Jaehaerys I. Since Jaehaerys had the proverbial heir and a spare in his eldest sons Aemon and Baelon, and Jaehaerys sent him off to the Citadel after the Prince showed a singular lack of interest in administration, women and martial arts (His only interest was books). Given his exceptionally bright mind, Vaegon eventually rose to rod and mask of an Archmaester. Vaegon never suspected that the crown might come to him. But as it happened, Prince Aemon died in a war. Jaehaerys passed over Aemon's daughter Rhaenys and made his second son Baelon the Prince of Dragonstone. Baelon already had two strong sons so succession seemed secured. That did not come to pass, Baelon passed away of medical causes. Had Baelon ascended the Iron Throne, his son Viserys would have been the undisputed heir. There the histories diverge. Some argue that Jaehaerys had asked Vaegon to leave the Citadel and take up his position as Prince of Dragonstone since his Grace had already established the precedent of passing-over grand-children in favour of his children. Others argue that he had merely sent for him to ask for his counsel. In any case Vaegon came. If the former narrative is correct, Vaegon refused the crown because of his vows. He did however advise his father to call a grand council of the realm to elect the heir from all claimants (Primarily the Old King's grandchildren), which Jaehaerys eagerly seized upon. So if we assume that Vaegon was offered the crown, he didn't think he could be released from his vows just because it turned out he was now the heir by proximity.
Second and stronger example would be Maester Aemon. His father Maekar was fourth son of King Daeron II. Since Daeron had his heirs in his eldest Son Baelor (And his sons Valarr and Matarys), he sent off Aemon (Third son of a fourth son) to the Citadel, famously commenting "Too many dragons are just as dangerous as too few". Nobody thought that a third son of a fourth son might become heir someday. But as it happened:
- Baelor died in a tourney accident.
- Valarr and Matarys died in the Spring sickness along with Daeron II.
Daeron's second son Aerys I took the throne and died childless. Daeron's third son Rhaegel had predeceased Aerys (Along with Rhaegel's own twins Aelor and Aelora). So as a result his youngest brother Maekar, the fourth son took the throne. Maekar had his own heirs in his eldest two sons Daeron and Aerion. Yet again, the two eldest Princes predeceased their father. And then Maekar died unexpectedly in a battle against Rebels. Once again, The succession was thrown in confusion due to multitude of claimants. Another Grand-council was convened. Daeron's daughter was passed over because of her sex and mental incapability. Aerion's son was passed over because of his infancy and fears of him inheriting his father's insanity. That left only Maekar's youngest son Aegon but the Lords refused to have that half-peasant (He grew up living among the smallfolk as squire of a hedge knight) as their King. The Lords offered the throne to Maekar's third son Aemon first, quietly. High Septon himself offered to free Aemon of his vows as a maester. Aemon refused and took the Black instead so that he couldn't be used by dissident Lords against his brother Aegon. In Aemon's case however, it does appear that everyone believed he could be released from his vows given the new developments and him emerging as the popular heir. Aemon himself seems to think otherwise and he took the Black apparently to shut that door forever. So perhaps Night's Watch's norms are more final in that sense than the Citadel's vows? Or maybe Aemon just wanted to put some distance between himself and nobility of the realm.
A third case (The only example from Night's Watch) would be that of Lord Commander Hoare. He was in command of the Wall while his elder brother King Harren 'The Black' ruled Isles and Rivers. Harren already had plenty of sons so succession seemed secure. And then Aegon the Dragon came and ended the Black line when Balerion burnt Harrenhal. The Lord Commander Hoare did not abandon his post because apparently as per the new developments, he became the heir to the driftwood crown. Nor did he abandon his post to avenge the destruction of his house even though he ten thousand swords sworn to him.
Conclusion
There seems to be no clear cut answer for this.
- George may or may not have implied that new revelations do not have any impact on sworn vows.
- There are no clear examples of people who joined orders of servitude while they were both heirs to something and were unaware of it. The only examples are in the cases where by unforeseen circumstances, they ended up becoming the heirs.
