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但如果是我想换new employer, 就不一样了，对吗？
-Correct. If you ONLY use the new TN to reenter US, it will override the old TN. But you may talk to the IO at the port of entry that you want to keep the old TN and ask the IO to update your old TN in the system when they issue you the new TN. If the IO agree to do that for you, then you may keep the old TN at the same time when getting the new TN. So, just talk to the IO to see whether it works or not.

多谢8老师您不厌其烦的解惑