Apple could write code that checks the hardware ID supplied and sign only that version. It's been sufficient to prevent hacking so far and is the major protection relied upon until the latest generation, so I don't think this is an issue at all.
An earlier comment with more technical detail: https://news.ycombinator.com/item?id=11141499
UDID lock is good enough for tying development builds to specific devices but is not an unbreakable guarantee the software cannot be run on another device.
Yes, if you can do that, then you can flash anything to the phone. When someone shows me that happening, then I'll believe in this technology.
Even then you cannot "flash anything" to the phone. But you can flash a build signed by Apple which hard-codes a UDID check.
For example changing the WiFi or Bluetooth MAC is not locked down and effects the UDID. Because the UDID was never intended to be a way to bypass the device encryption it was not designed with anywhere near the same level of care and sophistication as handling the encryption key itself.
I never suggested the UDID. Please don't ascribe a claim to me that was never made. SHA512 the serial and macs independently. Job done.
It is foolish to assume that the FBI would stop at this special version of iOS, especially given how they have been arguing and fighting to break encryption for over two decades.
edit, on icebraining's interpretation:
That's exactly right - once the difficult part about targeting an individual phone is finished, it's easy to take the iOS signing key and target any device you want.
Clearly it is not, otherwise they would have done this. They did not.
This is what I understood, at least, from the parent's post.
The question is can you change one devices ID to exactly match another. This might require hacking the BPP or finding a SHA1 hash collision, both of which are absolutely possible but not necessarily trivial.
Possible? Maybe, but if it was that simple the FBI could simply change the signing key. It's not, and they can't.