U9 moving to 5v5 – what changes first?

I’d start with the things 5v5 is giving them more often, not with a new formation. The smaller game means more 1v1s, more quick changes of possession and less time for a player to disappear. So I’d spend the first few weeks helping them get comfortable receiving under pressure, attacking or defending somebody directly and reacting when the ball changes team. Use lots of 2v2, 3v3 and 4v4 in training. You do not need to recreate a full 5v5 match every time. I would also expect the game to look hectic for a while. They are adapting to a different picture. If you try to organise the chaos out of it too quickly, you can miss the reason the smaller format is useful in the first place.