Rest assured, you are not being hacked or invaded. I, too, have seen security sensors revert back to HC/UC units after a re-install, so it's not out of the question that am update would do the same.
I'm pretty sure that a security type sensor has a HC/UC assigned to it internally in the software or hardware, as years ago, KDR discovered a way to access and change the HC/UC info as a back door access. You can read about his findings by searching the forum. He documented it well.
Just as a clarification to those reading this thread. House Code/Unit codes weren't being changed in the Original Post, but Security Sensors were losing their encrypted code and reverting back to a Standard HC/UC.
Just re-install the sensors, and go on, keeping in mind that an update to the software may or may not put you in the same situation in the future.
That being said, I'm guessing that your security type sensors count towards your total of 256 units installed in the software.