Can i use old approved H1B petition ?

[Rakesh_H1B_2017]

Employer A - Petition approved for 3 years in Feb 2016 (up to 2019), Stamped Visa (up to 2019) and started working.
Employer B - Petition approved for 3 years March 2016 (up to 2019) and not used.

After landing in USA, Employed A filed an H1B Amendment and now after amendment approval I797 has reduced validity of upto November 2017. So basically my Employer A will file an extension before november 2017.

Now my questions::
can i start working with Employer B using old petition. This has validity up to 2019? I have good terms with them still.
Do i need to inform USCIS anything?
I do not want to leave US now fearing stamping issues but i can go after a year. Is stamping important before starting a job with Employer B?

[pontevecchio]

Your good terms are not that relevant. They were required to revoke your H1 and in any case will need to file another non cap  H1 petition and get it approved .