I would rather that they eliminate Lang if they really have to, and keep Pym. Maybe, instead of just inventing the Pym particles purely for research, Pym invents them to save someone or break into a place, in much the same way that Lang went to steal the suit from Pym to do just that in the original story. So rather than it being "to steal an Ant-Man", it would be more "to create an Ant-Man."
I definitely don't want Lang as the inventor and partner of Jan in place of Pym. That would just be a slap in the face to Hank.