(This will all be ISP dependent, your ISP could certainly do things differently)
Assuming your ISP lets your cable modem work at different physical addresses, the same account will be billed as the Modem's MAC address is how authorization, capacity, and billing are handled.
I once swapped a weak cable modem at client for a stronger one I was using, to get them working. Years latter, the ISP accused us both of 'stealing' from the other.
I tried to remain child-like, all I acheived was childish.