F1 to previous H1B transfer

I was previously on H1B and recently moved to F1 to pursue another Masters. I am planning to complete my Masters in December 2017. My question is if I find a job and they file a new H1B petition then will the H1B become effective immediately OR I will have to wait till October 1.

In other words, Can I start working from December 2017 OR I will have to wait till October 2018 before I can start working. Note that I am not switching to H1B for the first time so I won’t need to go through a lottery. I will be switching back to previous H1B.

The new H-1 petition would be cap-exempt if they submit proof of previous approval. So it would become effective from actual approval date and not Oct 20xx.