If they shift to being paid hourly, then you *must* (it's the law; the employees can't opt out of it) pay overtime if they work more than 40 hours in a week. If they are paid salaried (and the salary meets the threshold; see below), they are only overtime eligible if they don't meet one or more the "tests" or criteria for exemption, such as the executive (or managerial) exemption, the administrative exemption, or most relevant for engineers (they most likely meet it) professional exemption, all of which you can find on the U.S. Dept. of Labor website. If the are salaried, meet the salary threshhold, and their job/duties meet one or more of the exemptions, they do not get overtime.Note that in January, the salary threshhold rises to $47,476 on December 1, 2016--that is, any salaried employee making less than that annually *will* receive overtime.