Thanks for your information. I applied for the tn visa twice already, and i have to do it again in coming september. When I applied it the second time last year, the offer letter didn't explicitly say that the contract was being renew. In fact I told my boss to treat me as a new hire on the offer letter. The officer at ins didn't realize I was working for the same company until he saw the company name on the old tn-visa which was attached to my passport. No questions asked, so I guess I'm gonna do the same thing this coming september, as long as there's no restrictions on applying for new tn-visa.