It isn't being demolished. There have been major renovations lately. I believe the left side of the building (former pre school classrooms and social hall) are next to be redone, so that might be why it looks like it's being demolished. The sanctuary and new spaces have been beautifully done to expand from a single congregation's space to a campus for Jewish life which multiple communities call home.
The prior answers are mostly correct but incomplete. The building at the Williams Street corner is scheduled for demolition at the end of April. It will be replaced by senior housing. All activities formerly in that building are now relocated to the renovated Sanctuary and the new construction adjacent to it.
http://kehillathisrael.org/ki-next-building-our-future/
Thanks! Your answer is awaiting moderation.