H1B approved, change of status automatically? OPT reinstatement?

Background:

  1. Graduated from college in May
  2. Received my OPT EAD card
  3. Future employer No.1 (start date in October) helped me submit H1B application and H1B approval letter expected to be issued in August
  4. Received a second offer from future employer No.2 --> Decided to join future employer No.2 in reliance upon OPT (knowing that the H1B tied to future employer No.1 will be wasted)
  5. Haven’t informed future employer No.1 of this decision (of not joining them) and the H1B approval is due to be issued soon

Question:
I heard that once the H1B approval is issued, my status may well be automatically changed to H1B from OPT, even if I have not started to work for future employer No.1 yet.

In that case, if I am to join future employer No.2 in October, I will not be able to rely upon my OPT (which has been changed since the issuance of H1B approval letter), unless an OPT REINSTATEMENT process is completed. Is this TRUE???

I am so worried.