Ah, but therein lies the rub.
Does one keep Dragoon skills once they part with the Soulstone or do they go back to being a Lancer?
.....
The answer is Lancer but if someone wanted to RP it that way I'd probably shrug and look the other way as well.
If that were the case, though, every trainer NPC would become a 'gladiator' or 'marauder' after parting with their soulstone but we see that's clearly not the case. They retain their Job specific skills.