A hoped-for replacement should not be treated as established qualifying work. Confirm the actual assignment and the immigration consequences of the change before relying on the original travel plan.
Sequence the decisions
First settle the business scope, then review eligibility and any required filing action, and then coordinate admission or status steps. If the employee is already in the United States, prompt individual advice is appropriate because employment and status facts matter. Neither an earlier approval nor a new client deadline guarantees that revised work can start immediately.
Sequence the decisions so that the immigration position is established before travel rather than after arrival, because the options narrow considerably once someone has moved. Where a replacement assignment is being arranged, the sensible order is to define it, assess it, take whatever step it requires, and then travel. Pressure usually runs the other way, since a business that has budgeted for a resource wants them in place.
Establish what the employee would actually do on arrival if the replacement work is not ready, and whether that activity is consistent with the basis on which they would be admitted. Where the answer is uncertain, that uncertainty is a reason to wait rather than a detail to resolve later.