The term used by the US is change of status (when changing to immigrant status, the term is adjustment of status.)
The form for changing from one non-immigrant status to another is the I-539. However, you will not file I-539, because, in that form's instructions, you will find this:
An employer must file Form I-129 on behalf of a TN professional worker who is currently in the United States.
So, if you want to change status, your employer has to petition on your behalf using form I-129. If you are outside the US, it is optional for the employer to file I-129; instead, you can apply at the border. This may be easier since the form is 36 pages long, and your employer may be reluctant to deal with more paperwork than necessary.
As a Canadian, you don't actually need to get a TN visa before applying at the border; you just show up and apply. You will need certain documents and a $50 fee; information is available from several web sites:
Among the requirements are:
- Proof of Canadian citizenship
- Education and employment records to establish your eligibility for TN status
- A letter from your prospective employer offering you a job, which must be listed in Appendix 1603.D.1 of NAFTA.