Can the lower cable be replaced on a rotor?

yes.

Thanks. How do you replace it? Does the rotor come apart?

You shouldnt have to take the rotor apart to change the lower cable, simply pop off the small plastic cap on the top of the rotor, line the hole up with the lower cable, and pull it up & out.
I think the original rotor may not have a cap, so if thats the one u got you may have to take it apart to change the cable. I think there is a lock ring on the bottom side of the rotor that you would need to pop out to dissasemble.

I have an early rotor, (not the pro rotor), and it does have the cap, too.

Iceman

My rotor is the earlier one too. I didn't want to tear it apart if it was not repairable. Thank You.

My early rotor also has the cap. I thought all ACS rotors had the cap to change the cable. Bad design if not.

the very first ACS Rotors did not have the hole/cap, and did need to be disassembled to change the cable. be careful not to damage the clip or seat for the clip when doing this (see woody's gold trick star interview on the "Joe Kid..." video ).