Start well before the expiration date rather than in the final weeks, since gathering updated evidence and confirming the role still matches the original basis takes time. Exact lead time depends on the route and current circumstances, so confirm a realistic internal deadline with counsel rather than assuming a standard window.
Work backward from the expiration date
Set milestones: when updated duty documentation needs to be ready, when the employer needs to confirm details, when the actual filing or travel needs to happen. Working backward from the expiration date, rather than starting once it feels urgent, leaves room to address any mismatch or missing document without the pressure of an imminent lapse in status. Add one further item to that backward schedule, and put it first: the confirmation you need from somebody outside the organisation.
A credential body, a licensing authority, a former institution asked to verify something — each responds on its own cycle and none of them will treat this as urgent. Requests of that kind should go out at the beginning regardless of where the checklist places them, because everything else in the plan can be accelerated with effort and these cannot. Hypothetical example: a pharmacologist's extension needs a registration confirmation that the issuing body produces in monthly batches, and requesting it in the first week rather than the sixth is the entire difference between a comfortable schedule and one that depends on nothing else going wrong.