Instead of changing frequencies, maybe it would be cheaper for theaters to put up shielding to block signals from coming in and out of the theater?

You cant block the signal. Eventually cell phone providers wont broadcast both cell and data separately and soon will be one signal. To block that signal is to potentially block calls to 911.

Kinison, you can block the signal by adapting a faraday cage into your building design. All those buildings with metal exterior shells are most of the way there... just make sure they're all linked and grounded.

Seems to me* it ought to be possible to somehow use the cellular network itself to make this work—every worn transceiver essentially a cell phone that connects to a conference call at the beginning of the show, where the stationary "phone" serving as call convener is hooked into the theater's audio system. Cell-phone audio probably lacks the native frequency range needed for a live show, but there ought to be a way around that with compression.

*Me, who doesn't know jack shit about this.