We upgraded a hotel spa system a few months ago. They decided to keep the current speakers so we just upgraded the amps and added a control system & touchscreens. In all but one room we provided an AUX jack that could be used to connect iPods/smartphones. The odd man out had its own local amp and mixer with an AUX jack that they wanted to keep. There is a hardwired A-B switch up on the wall at the back of the equipment cabinet that lets you toggle between distributed and local.
When we can we eliminate “user confusion” issues like this with new hardware and programming. In cases like this where we have to deal with whats already there i always create a custom manual or howto infographic. It gives visual cues and basic instructions for end users. I have probably created hundreds of these graphics and short manuals over the years.