Historic theatres with a horseshoe shape, the so-called baroque theatres, constitute an architectural heritage and are of enormous importance. For these reasons, this paper aimed to identify a single acoustic quantity index as part of an early-stage digitalized procedure to improve the management of the complexity of their restoration process, both better preserving the original room acoustic field and making it variable for new different uses of the theatre space. The procedure introduces technical policies and design tools in a BIM execution plan (BEP) and is supported by a digitalization that inserts the restoration of theatres into industry 4.0. This choice was taken according to the new design goals and the new Procurement Code Directives, which underlie the use of digital technologies in this context. The described acoustic single number quantity, called a “multipurpose index”, summarizes the main room acoustic goals for when a baroque theatre is used as a multipurpose space after renovation, specifying and better controlling the requested room acoustic quality for people other than acoustics specialists, such as theater directors, and more generally for the management. As an example of this possible transition, the case study of “Teatro Ristori” is presented.