If I was an hourly employee and then moved to salary in the middle of a pay period, can they take away my overtime pay?

There is no easy "yes" or "no" to this question.    Most likely your employer is governed by the Fair Labor Standards Act (FLSA).  That law required employers covered by its terms to pay employes overtime (1.5 times the regular hourly rate) for any hours worked over 40 in a seven day workweek.  However, there are some exemptions to that requirement for certain types of employees.  By way of example, professionals (lawyers, doctors, CPAs, engineers etc) are exempt from this requirement as are managers, some supervisors, some top level office administrators, commissioned sales staff, interstate truck drivers and computer programming employees.  But title alone is not the determiner, the regulations governing these exemptions have more detailed requirements.  You can learn mor about this reqirement and its expemptions on the website of the US Department of Labor - Wage and Hour Division. If you are qualified for one of these exemptions or became so qualified in the middle of a pay period, I am not aware of any restriction that would prevent your employer from moving you from an hourly to salaried status.  However, if you had already worked more than 40 hours that week, before the move was made, I would suspect that the employer would owe you for those overtime hours.  Also, be aware that is is a common mistake or even an intentional attempt to avoid paying overtime for an employer to misclassify someone who should be an hourly employee as a salaried employee.  If you believe that this is what is occuring to you, the USDOL website above will explain how to file a complaint.