I believe that while XD is in beta you can use it without needing a CC subscription, so it's free. This page seems to let you download it without needing to be signed in:
Whenever XD is released it may well then come under Creative Cloud, though it is possible they might charge separately for it, like they do with Acrobat and Captivate. My hope is that it will be included with any CC subscription, even a single app subscription. But then I'm not in charge of Adobe, so it may not work out that way!
The Adobe MAX conference is in the middle of October, I would hope for more news about the future of XD by then.
You do not ahve to change it yet! for now it is a free download as a beta!
So far no dates to end the Beta but might be a special announcement during Adobe MAX October 15-20.
Download and play! it is a great tool!
Hope this helps!
Thank you both very much!