Canadian Citizen – TN Visa Change of Employer

Dear all - please help me on the following issue. I am a Canadian citizen. With TN visa, currently I have been working in USA for 8 months for an employer say "A". The TN visa was issued for 1 and half years. Now I have got anothet job offer from a different company say its name is "B". So I need to get a new TN visa for my second employer "B". My question - how can I get the TN visa for my second employer? Do I need to resign from my 1st job before going to get my new TN for my second employer? Do you think that my new emploer can arrange everything for me? Or should I go to the Canadian border to get the visa? How does it work? Please answer.

You go to border, like you di fopr 1st TN, with new documentation.
You get TN, keeping old one until you resign from "A".
You can hold both TNs.

What if person who applies to new TN because of change of employer lives in USA and therefore does not maintain his Canadian address anymore. Would it cause any troubles?