Those two had history long before The Empire Strikes Back, and while we don’t officially know what Han and Lando will be doing together in the movie, it will be interesting to see what their dynamic was like in those early years.
Besides, even if Donald Glover turns in an excellent Lando performance, it’s likely most people will still think of Williams first regarding Lando.
It’s certainly understandable that Billy Dee Williams would feel weird about someone else playing a character that’s so meaningful to him.
That said, the fact of the matter is that if you’re making a movie about Han Solo’s younger years, passing up on using Lando Calrissian would be a wasted opportunity.