Here's the deal. E-file your your I-129 through the USCIS website. Make sure you do it before the expiration of your present TN. Once you e-file, it will tell you where your application was sent. You will receive a a case number right away and a request for evidence. Send the evidence along with a copy of what ever part of the I-129 they want.
The reason I am telling you this is because earlier this year I sent in an application by mail to switch companies. I sent everything to Vermont in May along with all of the evidence. While waiting for this TN to come in, my exisiting one was about expire. Rather than take a chance, I e-filed the extension and after e-filing it told me that everything went to Laguna Niguel, California (I'm in L.A.). Oddly enough the e-filed one was approved in 2 1/2 months but the mailed in one took over 5 months. So go figure. I think the Vermont center is so backlogged with different types of applications that they started having renewals go to the closest service center near the applicant. By the way, switching companies is actually considered an extension of TN status if you ar presently in TN status and not a new one.
