The biggest worry I have when making a change from exempt to non-exempt is that the employee will realize that they should have been receiving overtime. I would just be careful of how you state the case. Personally, I like to state the possitive side about them receiving overtime now. I would say something about a recent review in their job responsibilities has lead to....etc. I would also show the employees the job description for their position and ask for feedback before you make the change - just in case there are items they do that haven't been reflected in the job description. You might to review all job descriptions so it doesn't appear that you are choosing to review only a few jobs.
I would also state that the change does not change anything else (if true), and that they are still important members of the company. I would also speak with the supervisors to ensure that they do not treat the employee different, except that they are no longer exempt under the FLSA.