hey satt, while I am not a TKD practitioner, I do know and have trained with TKD guys presently and in the past, and so far what I have seen is that they usually have a smooth transition since they have experience in an existing MA already. They aren't as nervous, are open to discussion and criticism, are not afraid to ask questions, and eventually once they get a good base down, they are the most flexible people I have ever trained with. They like to use their legs equally if not more, which can be a good thing and a bad thing. Bad, in that, I know of one person in particular who tries to use his flexibility and legs to escape from almost any position by flailing them around to wrap up opponents. While this works on some people, it usually doesn't work on more experienced players, and usually pisses them off by stratching them in the head and face/upper body with toenails. Good, in that, they can create challenging situations out of basic principles using their flexibility.
Also, I wouldn't recommend practicing flashy kicks in a BJJ school. The instructors usually don't like that. But overall, from what I have observed, it can be a smooth transition, especially a good base for MMA if that is your goal. peace and safe training.