Here's the link to dpreview.com. They say "it's a completely new lens, rather than a revision of Sigma's existing 50mm F1.4 EX DG HSM".
http://www.dpreview.com/news/2014/01/06/sigma-announces-all-new-50mm-f1-4-dg-hsm-art-lens?utm_campaign=internal-link&utm_source=news-list&utm_medium=text&ref=title_0_0
Some more information is here: http://photorumors.com/2014/01/06/sigma-50mm-f1-4-dg-hsm-art-and-18-200mm-f3-5-6-3-dc-macro-os-hsm-lenses-announced/
MTF charts: http://sigma-rumors.com/2014/01/sigma-50mm-f1-4-dg-hsm-art/
http://www.dpreview.com/news/2014/01/06/sigma-announces-all-new-50mm-f1-4-dg-hsm-art-lens?utm_campaign=internal-link&utm_source=news-list&utm_medium=text&ref=title_0_0
Some more information is here: http://photorumors.com/2014/01/06/sigma-50mm-f1-4-dg-hsm-art-and-18-200mm-f3-5-6-3-dc-macro-os-hsm-lenses-announced/
MTF charts: http://sigma-rumors.com/2014/01/sigma-50mm-f1-4-dg-hsm-art/