She could change she'd just have to sit out fed cup competition for 2 years.
But in saying that, I would prefer her to stay belgian, I am an aussie and understand it would be fantastic for Australian Womens tennis but also realise the huge investments flemish tennis have put into Kim. Remember when Dokic left Australia and how Australia reacted now if Australia were to try and pouch Kim I would consider that very contradictory, given their comments and so forth about Dokic's abandonment. Also tennis australia understand how it is to lose a player they have invested so much in, so can't see them pouching Kim knowing how they felt when they lost dokic.
I think she will eventually obtain australian citizenship as it will be easier for her, but expect her to keep her belgian and european citizenships and play for Belgium. I don't think she will play Fed Cup next year, but remain representing Belgium.