Complex sentence: In 1938 the design was reworked by Jiri Jakub who converted the space under the church which had been intended as a theatre into a columbarium where funeral urns could be stored.
Split into simpler sentences: In 1938 the design was reworked by Jiri Jakub. He converted the space under the church which had been designed as a theatre into a columbarium where funeral urns could be stored.