Easiest approach first: Add a 2-day notice to retirement so we can look for replacements and have them ready when the time comes. A week would be even better (training), but either way we’d know when retirement comes before it actually does.
Secondly, the retirement notice could/should include some minor information about the retiree other than job and business. If it says how many hours of shifts they have at the moment, we can look for an appropriate replacement (full time/part time).
Thirdly, HQ employees are a different species entirely. When they retire, everything can break down. A logistics manager that leaves means you need to replace them, train them up and then try to recreate the 16 supply routes the previous one organized. That would be a game-quitting moment that is inevitable (even if you had a replacement ready). One solution that seems easy to me would be a message from a HQ employee when they turn 64 (or 60, if we want to be gracious) telling us that they are approaching the end of their tenure and training an intern up as a replacement. Their hourly wage would rise significantly but once they’re retiring, the replacement takes over without a hitch. In game this could even be handeled elegantly by simply renaming the employee and changing their age and wage while leaving everything else untouched.