1- There are conflicting views about this. As per American law, Attorney and even port of entry official you do not need visa stamping. Airline will allow you to board flight for USA , POE official will allow you to enter USA. Everything looks fine.
But for this , Consulate visa officers in india have a different view. They insist that since previous visa stamped was for previous employer - who no longer is your employer , you need fresh visa stamping.
Advantage with fresh visa stamping is you will have a clean record as per consulate and will get a longer validity visa. Disadvantage is that there is a risk of you getting 221 G for some reason.
Considering pros and cons it is your decision.
However my personal opinion is that you should go for fresh visa stamping.
Problem comes - if you are unmarried and want to take wife with you after marriage. In that case without fail both husband and wife should apply for fresh stamping.
If you have old visa and wife applies for fresh H4 she will get 221g