Changing jobs on OPT before H1B transfer

[spikedex]

I am working for employer A on OPT which will expire on Sept 30th, 2014. It was originally valid till May 27th, 2014 but has now been extended through cap-gap extension since my employer had filed an H1B on April 1st. The H1B has now been approved effective Oct 1st, 2014.

I now have an offer from Employer B. Employer B wants me to join them on OPT and start the H1B transfer process after I join. Is this legal?

Will it be safer for me if they file for my H1B transfer before I quit my current company? Is the safest option that I quit my current company only when my h1B transfer gets approved?

[rahul412]

DO NOT JOIN B WITHOUT H1 APPROVAL.