All excellent responses. One potential work-around which is becoming a better and better option is to file a petition with USCIS rather than apply in the first instance at the "border."
Ie, if you are currently within the US on a TN, and looking to switch jobs, you can not file for a new status with USCIS; your options are either to leave the country and apply at a POE, or leave the country, and only then petition with USCIS.
Is this incorrect? Can a current TN holder petition USCIS for a new TN while still within the US?
However, it's been a slow process, especially with having received word of USCIS requesting additional evidence from my first application.