Part time H1 to full time H1 convert

[love_u_akshay]

I am on my H1B visa. My initial petition was made for 32 hours per week part time job. It was approved, i got my stamping done at mumbai India and am currently in US working 32 hours/week. Now my employer wants me to work full time i.e. 40 hours\week. The job description, pay rate and benefits increase proportionately to the added 8 hours.
What is the formality needed to be done by my employer in order to get this done?
what documents are required?
how much time will it be before i can actually start working 40 hours?

Thanks in advance!