Unfortunately you can only upgrade to premium economy, business or first if you have at least paid for the most expensive economy fare (which may not be too far off from the business fare costs). If your employer is happy to book that for you, you are off to a laughing start.
There is a table detailing the upgrade costs in KF miles if you don’t already have access to it.
Since it costs less miles to upgrade from Premium Econ to Business and the costs is not too far from the most expensive Economy fare, it might be a good idea to buy Premium Econ (again, if your boss allows).
Before you go about paying extra, best to check award availability for the flight and class you are after. If there are no availability for outright award redemptions, you would not be able to confirm your upgrade immediately.
Hope that helps. Good luck.