You do not need to quit your current job to obtain a new TN visa. You can keep multiple active TN visas. You can go back to your original employer to work as long as the TN is not expired. This is the benefit of holding TN visas. You can also go to university for a degree while you are working using a TN. I had the same concern when I went to get my 3rd TN for the 3rd employer. The boarder officer ensured me that I could obtain my new visa, and then went to my old employer to work for 2 more weeks after I handed in the resignation letter. Don't worry!