But country A has 5 years to call him. Once he plays for country A (even scrimmage for U21 - it depends how it would be designed) he can't play for country B anymore.
for me the suggestion will only make sense, when they are considered for "national" in snse of marketing not NT, because this will lead to scary NT scrimmages and would be silly when you have a player for a long time and you want to gain the merch profit but it doesn't work because he played a meaningless game in the past.
And as i said, changing of the Nationality the place where you live is only a small factor.