Before the new duties take effect and before travel, not after the applicant has begun working under the revised description. If the reorganization lands close to an agreed start date, moving the start date is usually easier to manage than reconstructing what was done in the interim.
Review ahead of the shift, not after it
Review the change before it takes effect so counsel can assess both documentation and authorization consequences. Some changes require more than an updated description. Ask the employer when the revised duties begin, identify what must be resolved beforehand and keep the start flexible until the relevant assessment is complete.
The corrected offer usually waits on somebody at the employer who has other priorities. Only then fix the start date with the hospital, and ask it to date the employer letter close to the planned entry so the period requested is current. The immigration step is usually the shortest part; licensing and employer paperwork set the real timeline. A registrar's letter confirming that every requirement is complete is not a conferred baccalaureate, and entering as a visitor while the parchment catches up creates a worse problem than a delayed start.