Goffel,
Having perused Part 61 Subpart 5, which is up for comment on the CAA website, I still do not see the 5700kg restriction. This, to my understanding at least, means that effectively nothing changes. The 1900 is still certified for single crew ops which means that the priviledges of the CPL still allows for flying as PIC although 121 requires 2 crew, but only states that they must hold valid licenses and ratings (No mention of type of license ie ATP).