Before going too crazy based on assumptions I would check to find out what access will be allowed. At most events I've been to the photographers move around in the aisles and in front of the stage in addition to having access to the wings which, depending on choreography, can provide a wide range of lighting and other interesting scenarios. Other back stage areas include the green room, hallways, warm up areas, etc. Flash is usually allowed only in offstage areas and LED lights might be better received.
If the lenses you already have work for you at concerts what is it that you are wanting to change, or improve on, for this event? Are you thinking that the stage lighting will be really dim? Is it possible to show up during the tech setup, or onstage rehearsals, to find out what is going to be happening and preview the numbers so you can maybe plan out some shots?