Hi..The original G3 CloneCloth and the New Look CloneCloth are both fully compatible with all characters created using G3 avatars. Obviously if your characters were created using G2 avatars then you won't be able to mix and match parts apart from the heads. These can be interchanged between any generation of avatars.
One small issue you may find is that the head sizes of the New Look CloneCloth avatars are smaller than the original G3 avatars, but this can be overcome by resizing the head to get a better fit.