Lifetime and Drive Replacement

On the Premieres (specifically the Elite), is the serial number Tivo uses to track your subscription on the hard drive or on the motherboard? I was considering purchasing an Elite with lifetime service but was wondering what would happen if the hard drive failed, the unit was out of warranty, and I replaced the hard drive myself? Would the new drive be associated with the previously purchased lifetime service or would I have to repurchase the lifetime service?