You're flying around with old stuff that DOESN'T WORK???
If you have to do a major change to the panel, you will probably take out the old panel and put in a new one (remember, you will be able to make your own, it will no longer be a multi-thousand-dollar piece of aluminum). Don't throw it away.
If it's not a major change, you will be able to put the original junk back in. I would expect to see guys making kits to adapt the old radio racks to the new electronics. This sounds like a perfect job for Weird Jim -- a little hardware and a new wiring adapter, and away you go.
Since they haven't decided what the dividing line will be between go-fly and gotta-be-certified, we don't know why anyone would WANT to go back to the original rule, but I'm going to assume that it will be actual commercial operation -- and might not even include instruction (you can learn to fly in E-AB or E-LSA).