If I’m changing the Session Key, does it matter if I set the upper (KU) or lower (KL) first?
No it doesn’t matter, you can transmit in either order. When the
endpoint receives both the upper and lower half of the key it will
enable encryption automatically.
Also note that the KU and KL commands are only used to transmit the
session key from the coordinator to an endpoint. It is not used to set
the session key directly. To set the session key use the SK command.