hi Eric,
"You can always use the Reset serial number option to clear the serial number. This also erases the device as well. So there is a way to recover the device if you forget the serial number."

You can't do this if the security bit is set.
The chip accepts the "RESET" option(and erases the devise and serial number) but you can't programing it via ISP.
And in this case you can't reset security bit via isp.
So you lose isp functionality.

Regards
Lambros