Regarding your question, How many times can it be changed? I believe it can be changed countless times to suit your needs; as I have seen many people change theirs so many times I'm uncertain if there even is a set limit for the time being.
As for the other question- changing you in-game username will also apply to your forum username being changed to the same thing, so yes. Although what is currently not implemented is a way to change you in-game username without changing your forum username automatically.
