Thinking about it realistically, JB is due to play seven weeks today. Even if he cancelled today (and he won't, it'll be at least another three weeks if he does) and they announced a new headliner, it would have to be someone very close in style to him so they get as few day tickets returned as possible.
If they suddenly announced someone from an entirely different genre they'd have to handle the returns and then VERY quickly sell another ten, twenty, thirty thousand day tickets in three or four weeks - unless it was a mega-act, you couldn't do it. There's a reason gig tickets go on sale months in advance.
Therefore, I reckon Stromae being elevated to headliner is probably the only option at such short notice, with a 'name' support act added in.