Review the actual selected activity and applicable permission before starting.
Do not transfer assumptions between offers
A deadline or prior assessment of a different job is not government authorization for the new arrangement. Identify changed employer or duty facts clearly.
Once the signed offer letter exists, apply at the port of entry shortly before the start date, or have the college file Form I-129 several months ahead. If a credential gap has to be closed by study, that alone can take two or three terms, and the employer letter cannot be finalized until the start date and duties are settled. What goes wrong at this point is a calendar error with real cost: non-refundable flights and a moving date are booked around the start date in the offer, as though that date and the date work may lawfully begin were the same thing. Admission is not guaranteed by an offer or an approval, and a tenancy agreed around an expected start date is difficult to unwind if the timing changes.