You have to go to the SSA office and re-activate your old one, take your I-94 with you. You always use the same SSN, the only thing that changes is whether it is valid for employment or not, but you will always need it for example when you claim CPP when you retire, you need the SSN to sort out your totalization claim.
Most employers never bother to check whether the SSN is actually valid for employment or not, however the SSA (and consequently, the IRS) do. This may cause you a problem down the road because it might appear to the SSA you were working illegally. And in TN-1, ten years from now how do you prove you were in legal status? The I-94 will be long gone.
It also appeared to cause a problem with the
tax refund cheques that were sent out last year, if your SSN was marked invalid, you didn't get one.
Steve.