First of all, you don't volunteer for a downgrade. If it happens and you cannot avoid it, you offer the refund to your employer or whomever is reimbursing your travel expenses. However, if there aren't legal or tax implications, I'd like to think that a good employer would give the refund to the employee, who after all did suffer on the flight and hopefully was nevertheless sufficiently professional to perform well regardless of being tired from the flight.

Avoiding such situations (as well as IDBs when the employee is needed at the destination) is one reason employers should encourage traveling employees to participate in FF programs and, where realistic, attain elite status on one or more of the airlines that are being used for the business travel.