How Risky is to Undo Sliding Genioplasty?

It is not risky at all. The bone cut is easy to make and reversing the bony position is quite straightforward. I have done it a few times and not found it to be a problem at all. It is the hardware which can be little difficult to undo at times. The more critical question is what will happen to the soft tissue under the chin in the neck. But that would depend on how much bony movement was done and how long ago the surgery was performed.