Durham's SpyGate investigation is now officially a criminal investigation
I'm honestly not sure why this step is only being taken now; it seems likely there was more than enough available evidence before. My guess is that Durham and Barr wanted to keep the investigation a non-criminal one as long as possible for some policy or procedural reason, and this change has some policy or procedural reason to be happening right now.
First, Clapper and Brennan have been making noise in public about Durham wanting to interview them. Why would they make noise about that? The only reason that makes sense is that they think they are targets and would be at risk if they were interviewed. They are probably right about that; they are almost certainly targets, and sitting to an interview would make them sitting ducks for the kind of perjury trap that got Michael Flynn in so much trouble. So, likely they refused to be interviewed voluntarily, and they are no longer employed by the government, so IG Horowitz doesn't have any lever to make them agree. A criminal investigation can compel them to either talk, or plead the 5th.