The chairs were removed in an "incredible" two days this week, leaving the auditorium ready for carpet to be pulled up and floorboards sanded.
Where performers used to work, people in high-visibility vests and hard hats were working around the stage before months to be spent improving the performing and backstage areas.
The theatre will be closed for about eight months, in which time a new stage will incorporate a modern counterweight fly system for raising stage scenery, the basement and storage areas will be enlarged, new dressing rooms will be provided, and a larger orchestra pit will be built.
The box office and function rooms would remain open, Andrea Ford said.