Can i start my H1 in Masters quota before October 1

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Registered Users (C)
I have done Masters from US and will be applying for H1 in Masters quota of 20000 begining Oct 1 2006 (as last years quota has allready expired). My question is can i start my H1 before Oct 1 2006 (lets say April 1 2006) for a quota that starts on Oct 1 2006. I have heard from someone that students who have done there masters from US can start there H1 at anytime during there OPT (my OPT is valid till Nov 2006).

Lets assume i can't start my H1 before Oct 1 2006 in that case lets say apply for my H1 with company A on April 1 and get confermation by May and it starts on Oct 1 2006 now can i change my company before Oct 1 and transfer my H1 to Company B before Oct 1 (lets say in June 2006) before my H1 offiially starts (My OPT is still valid till Nov 2006).

Lastly if my H1 starts on Oct 1 2006 and i get my H1 papers by May 2006 can i go and get my H1 Visa stamped from Canada or Mexico before Oct 1 (for example in May 2006) or do i have to wait untill my H1 officialy starts on Oct1 2006.
 
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