Change from H1B to TN, when exactly H1B becomes invalid?

Hi,

I will be grateful if anyone can give me advice about this situation:

I was on H1B for company A. I got an offer from company B last week and I just visit POE to get the TN visa with new I-94 card. My TN support letter and offer letter clearly state my first date for company B is 09/07/2010, that is, 2 weeks from now.

The problem is that I assumed I could still work for company A(my H1B sponsor) for another two weeks, so, I planned to let my employer know the coming Monday, to fulfill my 2 weeks notice advance promise in the next two-week time. However, I just heard that my H1B would be void as soon as I got my TN visa, is that true?

That is, will the status change(from H1B to TN) start from the time I got the TN visa at POE(09/22/2010), or from the time I start my new work for company B on 09/07/2010? Is the H1B visa still valid for me to work for company A for another two weeks before I start my new work?

If my H1B visa really became invalid at this time, I will have to tell my manager at company A the true and apologize for that, although I feel very bad about that. If not, I worry I would be caught by the government and charged for illegally work for two weeks, then I will be in big troubleshoot for further immigration petitions(TN, H1B, even GC, etc.).

Please advise and if possible, please provide references with which I could show my H1B sponsor/employer.

Thank you very much!

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