I think i know what your getting at, where you get limited or no connectivity after swapping MACS even when its valid. Just change your ethernet mac and the modems serial to the same as the hfc mac (without the colons). Reboot the modem and wait a couple mins for the new lease.
If you can't get onto the modems web gui just force your local ip to 192.168.100.X and set the gateway to your modems ip, but remember to change it back to automatic after you confirm the mac is "Operational".
If you can't get onto the modems web gui just force your local ip to 192.168.100.X and set the gateway to your modems ip, but remember to change it back to automatic after you confirm the mac is "Operational".