Complex sentence: During its conversion into the MSI, the building's exterior was re-cast in limestone, retaining its 1893 Beaux Arts look, while the interior was replaced with a new one in Art Moderne style designed by Alfred P. Shaw.
Split into simpler sentences: During its conversion into the MSI, the building's exterior was re-cast in limestone to retain its 1893 Beaux Arts look. The interior was replaced with a new one in Art Moderne style designed by Alfred P. Shaw.