The documents would be the same as what you had given for your first H1 with an addition of the approved I797 from your current employer. If filled under regular it can take from 2-6 months and if filled under premium processing you would get the result in 15 calendar days after filing.
Saurabh, to the above reply what if the previous employer retained the petetion that was filed by them can we still go ahead with the h1 transfer process in any other way ?