I think they made a mistake, but...
...here's to hoping they can pull it off (they being the filmmakers and Ehrenreich).
A lot of people have already pointed out the 5 inch height difference between the two actors, which is substantial when you're talking about such an iconic character. Yes they can use camera tricks and different things to add height to him, but in the end he's a small fry version of Han.
I liked Ehrenreich in Hail, Ceasar! but I'm not sure how well he'll be able to portray Han. He has to do an impeccable job, or else be torn apart by the wolves because fans aren't going to give him an inch of wiggle room here.