What happens when we run out of ref numbers

Recently we brought up in discussion that we don't know what happens when incidents or test cases reach the point of TC999999,

Understandably this wont be for a while but with the way we work we have found we are moving through these reference numbers rather quickly. When this occurs what will happen, will the numbers cycle back to TC000001?

If so what would happen to the original TC000001?

Let me know if you need any more details.

Thanks,

Ben