changing job and changing TN, advise?

I am changing a new job, for which I need to get a new TN. I have some questions here.

Before I get my new TN, I still work for current employer, when I get new TN, the CBP officers immediately take my old TN back and issue new TN, but I did not talk to my current employer or give them notice yet (of course that only be happened after I get new TN), in that case, with new TN, I can not work for old employer anymore (say 2 weeks notice), how to handle the two weeks? Another thing is if CBP declines new TN, will he retrieve my old TN too? I mean if he retrieves my old TN, how I can work for old employer?