Complex sentence: In the early 1990s, it became apparent that more work was required in the theatre if it were to continue to serve the public into the next century so between October 1999 and June 2001 an extensive program of both internal and external restoration took place.
Split into simpler sentences: In the early 1990s, it became apparent that more work was required to permit the theatre to serve the public into the next century. Therefore, between October 1999 and June 2001 an extensive program of both internal and external restoration took place.