Can we transfer H1 visa 2015 before Oct 2014 , currently in H4

I worked in L1 for my old client then staying in H4 now and currently got H1 aproved from a diff consultant (Let say A) for this yr 2015.

My old client is ready to take me back through a consultant B, so please let me know if I can transfer the consultnat from A to B before oct and what is the earliest day to start and will i be under cap-exempt or not…