Can't select player even considering that he already played for me

I don't know the specifics of when a player can change nationality myself, I had personally thought it was if a player had been in non-competitive games it was fine. However, the QA team will review this further.

I've seen it occur for a San Marino player who had appeared in competitive qualifiers

While the rules aren't necessarily the most clear, if players are under 21, eligible for both nations at the time of making their first appearance and have played no more than 3 games - while also not making an appearance at the finals of a tournament they can switch. 

So there are a lot of conditions under which it can happen, it's one of those areas that has become more flexible for younger players than in years gone by. 

Thanks for clarifying, @Michael Sant.

I believe we should document that, just in case.

Btw, if anyone wants to avoid this, please don't remove that player from your squad until he makes 4 games.