H1B Petition is valid till Sep 2015. Can I transfer H1 after petition expire?

[BR63]

Hi

My H1B visa is valid till Sep 2015. H1B petition is also valid till Sep 2015.

What happens if I don't travel to USA till Sep 2015 and my employer doesn't file an extension for me?

Can I apply for an extension with the new employer after my petition validity or my Visa expires i.e. Sep 2015 or I will need to file an extension first from the current employer and then I can transfer it?

Will I be able to get new visa stamped from a new employer based on the same petition with expired validity?


Thanks