An internal approval does not grant immigration permission. Review the actual TN process and authorized arrangement before relying on the start.
Separate company and government decisions
A completed HR workflow is not an admission or status decision. Canadian citizens may seek TN classification at a U.S. port of entry or through an employer-filed Form I-129 and generally do not need a TN visa stamp.
If the engagement is expected to continue past it, start gathering the employer's updated letter and any refreshed evidence weeks ahead, since collecting documents from a different country on short notice is what most often causes a gap. Meetings booked around an assumed start date create pressure to proceed on assumptions, and moving a calendar entry is far easier than undoing an activity already performed. Where restructuring is required, the limiting factor is usually a board or shareholder approval process with its own meeting calendar, so ask early when the next opportunity to approve a change falls. Where the employer needs cover in the interim, that is a staffing question to be solved separately rather than a reason to compress the immigration one, and saying so early usually produces a better conversation than discovering it after a second attempt.