In the sticky it says

Q: Do units, like Wood Elves, lose any damage and resource tokens on them when they become a development?
A: Yes, that is correct. However, if the card is also a unit then unit rules take precedence and it can have damage/tokens on it for as long as it is a unit.

What official rule supports that? What I have seen on other forums (including fantasyflight one) half believes this and other half opposite.

Also, what about attachments? If I have attachment on tree kin and turn it into development does it go away?

ok nevermind, just got answer from Fantasyflight that its gonna be clarified in the next FAQ. Concerning attachments they should be also removed.

fantasyflight forums