The linked question is wrong. When Player’s Handbook II states that you lose a feat, it means you lose every benefit you’ve gotten from that feat (aside from like, XP and items you gained as a result of adventuring with that feat). The whole point of the retraining rules is to allow you to make different choices that you could have made to begin with—not to enable new options where you can keep your cake and eat it too.
No single, simple, ruleset is ever going to cover every possibility in a game as large and as complex as 3.5e, and Player’s Handbook II doesn’t even try. Instead, it leverages one of the greatest assets D&D has: that it is run by a thinking human being. The DM is required to handle the retraining from start to finish, and that includes making judgments about what losing one thing and gaining another really means. But the overall guiding star for that process is the idea that the PC ends up in just the same position they would have if they had simply made their new choices the first time. This is not always straightforward—and may sometimes be impossible, or more damaging to the campaign than it’s worth—but that is the stated goal.
That being the case, pretending that you “can’t” undo events that occurred at the time the original feat was taken is, bluntly, against the rules: that does not achieve the stated goal of the retraining. The rules instruct the DM to go beyond that, and, for example, remove things learned by taking the feat. That the retraining rules don’t explicitly spell this out is irrelevant—because they explicitly tell the DM to figure this out.
I am generally in favor of being precise and accurate with the rules. I am entirely comfortable with rules lawyering. But this isn’t that—this isn’t strictly applying the rules, it’s selectively picking some rules to take over-literally while outright ignoring others. If you want to make a case on a technicality, then you have to handle every other technicality that might be involved; you don’t get to ignore some because they get in the way of what you want.
So the too long, didn’t read here is, Player’s Handbook II does not support any kind of gaming of the retraining rules to get extra benefit, and calls on the DM to make sure none happens. In this case, that means the PC loses knowledge of the stance when they retain Martial Stance, and when they lose knowledge of the stance, the stance ends for them. Any other ruling is inconsistent with the rules of retraining and therefore invalid.