Look at it like this:

When you had the discussion about the different ways you could do a system for them, they chose the cheapest route. And it was a be all/end all setup with no expandability.

Now they want to add to the system. They should have took your advise to start with and went with the expandable system.
Now they will have to pay for a redesign & installation. There really is no way around this. They chose this path, either they pay or they don't play.

They are lucky that a system like Ethereal's exists to help with a cheaper solution.
Until they want to do video walls, PiP, audio breakouts, etc... Then another system redesign & installation with a full matrix or JAP setup will grab at their checkbook.

If they don't know what they really want to start with. And can't be bothered to put in the effort to think about it ahead of time, then this is what they get.