As for when an employer can shift or change an employees classification from non-exempt (e.g. hourly employee earning overtime) to exempt, there are only two legal occasions for that:
First, if the employee has been misclassified to begin with, the employer is allowed to correct that misclassification. That is, if the employee really should be an exempt employee and it was wrong to classify him or her as non-exempt, the employer may start paying him or her on the correct basis.
Second, if the nature of the employees job changed, so that a formerly non-exempt employee now truly is exempt. For example, say that someone was a manager without a staffin all ways, he or she met the test for being an exempt executive (managerial) employee, save for not having staff reporting directly to him or her. However, the company has expanded and hired, and now that person is supervising at least two full-time staff or their equivalents. In that case, that person would now meet the requirements for exempt executive and could be paid on that basis.
However, the important thing to remember is this; employers cannot arbitrarily change an employees classification. An employee must truly be exempt to be treated or classified as exempt.