H1B transfer with my previous employer

[sukurams]

Hi,

Someone please clarify my queries with respect to H1B transfer to my previous employer:

I have joined with new employer Company B during dec'16 from Company A before 2 months only.
Now i would like to join back Company A again in mar'17. Still i do have a valid I-94 with company A and my employer (Company A) agrees to recruit me back.

Please clarify,

a) Should my previous employer (company A) must initiate fresh H1B transfer?
b) Is there any time frame set for initiating H1B transfer once again with one's previous employer?

Is there any difficulties seen by anyone, please share.

Thanks