can i change my employer in CAP- GAP period?

I filed multiple h1b petitions from two companies respectively A &B. Currently i am an employee of Company A. Petition filed by Company B has picked in the lottery and my job start date is oct 1st 2017. Can i work under Company B before the start date? or I have to work in company A till october 1st. If i change my employer as a company B what are the advantages and disadvantages of it? what happens if i change my employer in this period. I called my DSO about this thing they are not sure about this issue. Please advice me. If i change my employer to company B does it improves any chance of Approval?