Currently the game works so, that it will not transfer free items to existing accounts (account that has been used to play Angry Birds Friends earlier). When you play on Guest account, all Bird Coin rewards are made "free coins". Power-ups bought entirely with free coins are made "free power-ups".
When you are purchasing with Bird Coins, free Bird Coins are used prior using any non-free bird coins. If a purchase exceeds the amount of free Bird Coins, non-free Bird Coins are consumed to satisfy the remaining amount. If a purchase consumes one or more non-free Bird Coins, the purchased item is marked as non-free.