Provide the new arrangement for assessment before relying on the earlier file. A different assignment may alter duties, employer relationships or the basis for the proposed profession.
Keep the start date conditional
Ask the agency how it will communicate changes and preserve the revised documents. Confirm any additional immigration steps before work begins. A commercial substitution should not be treated as automatically covered by the original TN plan or admission.
TN is granted for a period not exceeding three years and can be extended or a new admission sought, but each renewal is decided on the duties as they then stand, which on a long construction project are rarely the duties described at the outset. The immigration step is usually the shorter one; an admission in TN status can be sought once the letter and credentials are ready, and admission is granted for a period set by the officer, with extensions possible while the employment continues. Field seasons create their own pressure: an employer that needs someone in the woods for a specific window will resist a slipping start date, so agree the schedule inside the offer rather than around it.