Changeable
The format is not as changeable as the actual codes.
If Russia or China is more interested in disrupting a launch, then causing a launch, they could make fake codes, and replace the real ones with the fake ones. Then if the President wanted to launch, he would be launching with fake codes, and it would delay a launch by hours as they figured out what was happening. To do this, they need real codes, even out of date codes, to copy.
Even if they want to launch, understanding what the codes look like would help. Understanding the codes could also help in disrupting the launch messaging system.
And that is just off the top of my head. This stuff is top secret for a good reason. I see no reason to sell it to our enemies.