H4 to H1 - Stay eligibility query

[Sri_USA]

Hello,

I am on H4 currently residing in US and have filed for H1 this year. My petition got approved and the recent case status shows RFE . I have 2 months left on
H4 and am planning to upgrade my H1 case to Premium . Here are a few queries I have :

a) Once I obtain my Approval Notice copy, am I required to go to my Home country and get Visa stamped to begin my employment here in US ?
b) If the answer to the above question is Yes, then how long am I permitted to stay in US with Approval Notice copy alone ?

Thanks in advance ,
Sri