H4 to H1 time processing if H1 is hold previously

Hello ,
I was previously having H1 visa. As i didn’t get new job after my job contract was over I converted it to H4 (H4 valid till Dec 2014).

Currently I am getting a job opportunity and want to convert back H4 to H1.

So my questions are :

  1. Time required to convert H4 to H1 in case of premium and Regular H1 visa process.
  2. Can I start working once I receive the acknowledgment receipt of filing H1 visa or do I need to wait for approval .
    I am asking this because my employee wants me to join within 7-10 days.
  1. If you are going back to old employer who already has an approved unexpired petition for you, then it will just be COS; else it would COS + cap-exempt petition (aka H-1 transfer). The former doesn’t allow premium processing and can take 2-3 months; while the latter allows premium processing (15 calendar days, regular processing can take 3-5 months). It doesn’t include any time spent if RFE is issued

  2. You will have to wait for COS to get approved before starting to work on H-1.

Fastest route is ask them to file H-1 cap-exempt + COS with premium processing. 7-10 days may not be possible if they have to get LCA approved etc.