Will agent url’s from a production imp be unique? I had been told that they may change but would that url ever be re-used?
No, it will not be reused.
The URL stays the same for a particular imp blinked up onto a particular account (eg if you moved a device to another account, it’d get another URL, but moving it back would result in it going back to the original URL).
Commercial devices by default get a new URL at every blinkup, though customers can choose to just keep one if needed.