Typically, unrestricted Y is much more expensive than the cheapest biz class fare. Depending on the route, there can be a several hundred dollar difference between the cheapest coach fare and an upgradeable coach fare. If the OP books an upgradeable fare but only seeks reimbursement for the lowest available fare at the time he/she booked, then there is no ethical issue (in my view). However, if he/she seeks reimbursement for the more expensive coach fare simply to get the chance to upgrade, that would be an ethical violation and likely would violate the company's travel and/or business conduct policy.