Our son is approaching his sixth birthday and third year of beinbg adopted. We're both very open about it and he knows his status: he is loved, safe and a key member of the family. The fact he had different birth parents is irrelevant at the moment. How he adapts to that is uncertain.
He does ask a little about his past and we're pretty straight with him, sometimes reading his life book. I ensure I never bad-mouth his parents, as they were the victims of a poor environment.
How was it different? The ease of transition from his foster parents. His first night was a little difficult, but there was little pining and, due to his rough background, he must have just accepted it. Which is a concern.
How is he now? Academically fine, he is now average height and weight, likes climbing, swimming, MMA, running and cycling as well as having a deep fascination with bacteria 😟
However, I suspect some behaviours are manifesting. He has bonded extremely closely with the husband and makes him take his shoes and socks off at night to ensure he (husband) isn't going to run away. Son does lack a little security and still asks if he is dreaming as he is so happy. He is demonstrably frightened of it being taken away.
Look at it form their point of view: you have essentially taken them away from one family against their wishes. That we cannot dispute, so it is up to you to prove to the child it was the best move. Whatever your motives are for adoption, they (probably) will not see it that way. That's fair enough.
Behaviour wise he is much better than most kids of his age. He is helpful in school and really good at making the younger kids in climbing feel settled. He is beautifully caring, with the odd outburst. We discuss this using 'together-time' and chat about feelings. I think most of stems form insecurity.
If you want to hear horror stories, I cannot give them. It is beautiful to know he is 'sleeping' 8 metres away in his room and while there are some rough days, they disappear when you see his peaceful sleeping face.