From https://www.tievskylaw.com/services/employment-based-non-immigrant/nafta-professionals-tn/
Change or Addition of Employers
There are two ways for an individual in TN status to change employers or obtain authorization for a concurrent position:
- through the employer filing a petition with the USCIS, or
- by returning abroad and applying on re-entry.
Termination of Employment
Upon termination of employment (whether voluntary or involuntary), the TN professional may remain in the U.S. for up to 60 days, or until the end of his authorized period of stay, whichever is shorter). There is no requirement for the employer to pay transportation costs. If the TN finds a new job, he must recommence the applicable TN process from the beginning.