Can I work prior Oct 1 when I transfer from an approved H1B with company A to a newly petitioned H1B with company B?

Following situation:

I’m on a J1 visa which expires in June 2016. My current employer is willing to apply for an H1B for me in April. The official start date for H1B visas is October 1. Unlike F1 visas, J1 visas do not qualify for the “cap gap”.

So if I applied with my current employer company A for an H1B in April and get approved in May (premium processing) but then seek employment with company B via H1B transfer and company B successfully submits my H1B transfer petition, say in June, when can I legally start working for company B – in June upon successful H1B transfer petition or do I still have to wait until October?

I assume you have already taken care of the J-1 waiver requirement (if any).

Even if you get B’s petition approved, you cannot work for them until Oct 1, unless one of the employers is a cap-exempt employer like hospital, govt organization, university etc.