H1B-B2-B2-H1B

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I have one year remaining on my first H1B. I had done a change of status to B2(two 6month extensions granted). I applied for a third extension on July 15th 2011(status pending). I now have a company willing to transfer my H1B. Do I cancel my B2 pending extension after my h1b transfer receipt notice or after I have an approval ?

Thank you in advance for your response.