H1B Approved for 1 year need help on 797 Expiry


I got H1 approved for 1 year and its expiration date is July 2014.My Question is as follows.

1.If i chg  my employer will new 797 have 1 yr approval or it may vary based on new employer amended petition. 

2.When i am working for comp A.And my wife got H4 for company A. But due to some reason she is in India. Now i am changing company A to B. And i want to know Does she need to stamp H4 for Company B before coming to USA. 

Thanks in Advance