It's not just a CEO who can change that. It needs all kinds of changes on all kinds of levels. For instance if department leader's KPIs and bonus structure hasn't changed, they will continue to actively fight open source. Even if there is now a new, fancy department with even big names from open source communities, that doesn't mean all the other departments changed.
Also what happens to the devs who are there for 20+ years now, probably quite a few of them in senior dev and architect roles. Will they drop all that they've grown up with? Can Microsoft afford to let them go for big money and hire new, unknown people instead?
If they manage to get this github integration right, I may start to think about them in a positive fashion but until now there's too much doubt and too many things that might go wrong. Compared to the size of a huge corp we've only seen fluff up till now.