There is normally an extension of the opt to cover this period of time commonly referred to as the "CAP GAP". Speak with your DSO and employer's attorney.
Attorney Robert Brown's (former INS Director, 1972-99) reply to your question is general in nature, and does not constitute legal advice as all facts are known to him. For specific advice or representation you should retain an attorney experienced in immigration law. Mr. Brown's reply on AVVO does not create an attorney/client relationship not constitute legal advice.
As the new employer in its H1B petition will probably seek to provide evidence that the beneficiary is maintaining status when requesting extension of status, it is recommended that you be employed for at least a couple of months with Employer A before Employer B files its H1B petition requesting extension of status. Discuss with your current immigration attorney.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter, nor should it be viewed as establishing an attorney client relationship of any kind.
Until you are in H1b status, you cannot change employers. However a new employer can file for you before October 1st, and you can start with them once it is filed. It is correct that you must show that you are working with the first employer in order to take advantage of H1b portability. Since you already know you do not intend to work for the first employer and it is not yet April 1st, you may wish to ask the new employer to file for you on April 1st and avoid the risks.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.