I read a comment from Djokovic (I'm guessing it was translanted from the Chinese press) that the surface seemed slow to him and he hoped it would get faster. I haven't heard that they changed the surface so maybe it's just Djokovic not having played on it before and needed to get used to it. I hope it hasn't been slowed down.
The organizers said they didn't change the surface at all. It is exactly the same surface for last year. Djokovic is the first to practice on the surface which was ready to be used only a couple of days ago. That may be the reason for the comment.
Roger didn't yet practise on that surface, cuz the stadium is far away from where he lives. Instead he practised in a stadium close to his hotel which the organizers specifically prepared for the players using exactly the same surface. BTW, nobody was allowed in there.