I don't think it's so much that Brian is "known"--a bit more to the Broadway community than Jason, but his name wouldn't affect sales, but he did the very first workshop as Freddy. Apparently he was very good, so it makes sense that they would bring him in to replace Norbert.
Jason was great, but if you really want to talk about an understudy who should have taken over...Joe Cassidy! He was so great, had the little tummy(I agree about Jason needing to gain a bit of a tummy), and like was said about Brian, I think he had a better voice than Norbert. An excellent high range--I loved the high note he reached in DRN.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli